Basic brake system
Brake hydraulic systems
Compensating-type master cylinder
Centre-valve master cylinder
Valves in the hydraulic system
Hydraulic brake fluid
408 part four running gear
A braking system consists of two main sections. These
are the brake assemblies at the wheels and the
hydraulic system that applies the brakes.
The system includes the service (foot) brakes for
use when the vehicle is being driven, and a parking
brake, usually hand operated, which is applied when
the vehicle is parked.
Some systems have disc brakes at all four wheels,
some have disc brakes at the front and drum brakes at
the rear, others have drum brakes at all four wheels.
Basic brake system
Figure 24.1 shows the arrangement of a hydraulic
braking system. The parts are as follows:
1. Brake pedal – operated by the driver.
2. Brake booster – makes the brakes easier to apply.
3. Master cylinder – provides hydraulic pressure.
4. Caliper and discs – slow or stop the wheels when
the brakes are applied.
5. Brake lines and hoses – connect the master cylinder
to the brake calipers at the wheels.
6. Brake fluid – transmits force from the master
cylinder to the calipers at the wheels.
When the driver pushes the brake pedal, force is
applied to pistons in the master cylinder. The pistons
apply pressure to the fluid in the cylinder and the brake
lines transfer the pressure to the calipers. The pistons
in the hydraulic cylinders in the calipers are moved to
apply the brakes.
When disc brakes are applied, brake pads are
clamped against the disc. When drum brakes are
applied, brake shoes are expanded against the inside of
the brake drum. These are different actions, but they
both provide the friction between the parts that is
needed for braking.
A moving vehicle has energy which must be
absorbed by the brakes when they are applied. The
energy is converted into heat as a result of the friction
between the braking surfaces. The heat is then
dissipated into the brake parts and into the surrounding
atmosphere. Therefore, the brake pads and discs or the
brake linings and drums, together with their associated
parts, must be able to withstand high temperatures as
well as high pressures.
The hydraulic system is designed not only to transmit
force, but also to increase force. It does this by having
cylinders and pistons of different sizes.
Operation of the hydraulic system is based on a rule
of science which simply says: ‘Pressure applied to a
liquid in an enclosed space is transmitted in all
directions without loss.’ This is known as Pascal’s
Arrangement of diagonally-split brake system
chapter twenty-four brakes
Liquids will not compress
Force and pressure
For all practical purposes, liquids are not compressible,
and so hydraulic pressure represents a pressure applied
to the liquid. It does not mean that the liquid is reduced
in volume, as is the case with a gas.
Refer to the cylinder in Figure 24.2, which contains
liquid and a piston. As a result of the force on the
piston, the liquid applies pressure to the walls and
bottom of the cylinder. The piston does not move
because there is nowhere for the liquid to go.
Figure 24.4 shows two cylinders of the same size
(diameter). Force applied to one piston is being
transferred hydraulically to the other. Because the
cylinders and pistons are the same size, the force
applied to one piston will be the same as the force
delivered by the other. However, by having cylinders
of different sizes, forces can be increased or reduced.
Force applied to a liquid transmits pressure
■ Gas can be compressed but, for all practical
purposes, a liquid cannot be compressed.
Air will compress
If a cylinder contains both air and liquid, a force
applied to the piston will compress the air and reduce
its volume and the piston will move down the cylinder
(Figure 24.3). When the force is removed, the piston
will return to its original position.
This arrangement of air and liquid would be
unsatisfactory for operating hydraulic brakes, as the
force would not be transmitted through the system.
Much of the brake pedal movement would merely
be used to compress the air without applying the
■ When work is done on hydraulic brakes, air can
enter the system and the brakes have to be bled to
remove the air.
Force applied to the piston compresses the
air but not the liquid
Force is transmitted by hydraulic means from
one cylinder to another – equal cylinders,
Figure 24.5 is a simple hydraulic brake system with
three cylinders of different sizes. The diagram can be
used to explain force and pressure.
When the brake pedal is pressed, the force against
the piston in the master cylinder (A) will apply
pressure to the fluid. The pressure will be the same in
all parts of the system, but it will have a different
effect on the pistons in the other cylinders. There will
be different forces from the pistons as follows:
1. Cylinder (B) is smaller than (A), so the force from
(B) will be less than the force applied to (A).
2. Cylinder (C) is larger than (A), so the force from its
piston will be greater than the force applied to (A).
In an actual hydraulic brake system, the master
cylinder is smaller than the wheel cylinders, so the
Basic principle of a hydraulic brake system
410 part four running gear
force at all the wheel cylinders is increased. The force
can be varied by the use of different-sized cylinders
and it can be varied from front to rear to provide better
braking. This can be done even though the pressure is
the same in all parts of the system.
Example of force and pressure
To understand how forces can be increased, reference
can again be made to the arrangement in Figure 24.5.
The area of the piston head in cylinder (A) is 80 mm2,
in (B) it is 20 mm2 and in (C) it is 160 mm2. The area
of the piston head needs to be known, because pressure
acts on the surface area, and the force on each piston is
related to its area.
If a force of 100 N is applied to the brake pedal by
the driver, lever action will increase this to 800 N at
the master cylinder pushrod. This will produce
pressure in the system. This pressure, by acting on the
pistons of the other cylinders, will be converted back
to a force.
The force at (B) will be 200 N because the area of
its piston is one-quarter the size of (A) and the force at
C will be 1600 N, because its piston is twice the size of
■ Pressure is the same throughout the system, but
there are different forces at the pistons.
Brake hydraulic systems
Brakes are designed with a split hydraulic system. This
is a type of dual system that has a tandem master
cylinder with two pistons. The hydraulic system is split
into two hydraulic circuits (parts), each operating the
brakes at two of the wheels.
Some systems are split between the front and the
rear, so that the front brakes operate independently of
the rear brakes. Other systems are split diagonally, so
that there is one front brake and its diagonally-opposite
rear brake in each of the hydraulic circuits.
■ Split systems are a safety feature which prevent
complete loss of brakes. If fluid is lost from one
part of the system, emergency braking will be
provided by the other part, but only on two wheels.
Braking will be even at the front, but the rear of the
vehicle will be unstable.
If a failure occurs in the front brake circuit and not
in the rear brakes, then the rear brakes will provide all
the braking. The vehicle will be more stable, but
braking will be less effective than front-only braking.
In the diagonally-split system that was shown in Figure
24.1, the primary part of the master cylinder operates
the left-front brake and also the diagonally opposite
right-rear brake. The secondary part of the master
cylinder operates the other two brakes as a separate
With diagonally-split brakes, a failure in one part of
the system allows two diagonally opposite wheels to
provide emergency braking. This provides reduced but
reasonably stable braking.
Figure 24.6 shows a simple master cylinder connected
to a wheel cylinder of a drum brake. This has only one
piston, but dual, or tandem, master cylinders are now
The basic system shown operates as follows:
1. The system is full of fluid, being supplied from
the reservoir through the inlet port and the
Front–rear split systems
In a front–rear split system, the front brakes are split
from the rear brakes. If a failure occurs in the rear
brake circuit, the front brakes will continue to operate
for emergency braking, but braking will be reduced.
Basic master cylinder connected to a wheel
chapter twenty-four brakes
compensating port. When the pedal is pressed, the
piston is moved on its downstroke. This closes off
the compensating port and traps fluid ahead of the
piston. Fluid is forced past the check valve in the
end of the cylinder into the brake lines. Fluid
displaced into the wheel cylinder moves the pistons
apart and brings the brake shoes into contact with
the brake drum.
2. When the pedal is released, the master cylinder
piston is moved back by the return spring. Fluid
now flows from the wheel cylinders towards the
master cylinder and is returned to the reservoir via
the compensating port and the inlet port.
Tandem master cylinders
There are a number of variations in the design of
tandem master cylinders, but Figure 24.7 shows the
basic arrangement. It has a cylinder with two pistons,
which are referred to as the primary piston and the
secondary piston. A reservoir on top of the cylinder
supplies the brake fluid.
The diagram shows the position of the pistons with
the brakes released. There is fluid in the reservoir and
in the cylinder. When the brakes are applied, the brake
pedal moves the pushrod and this pushes the primary
piston along the cylinder bore. This closes off the
compensating port and creates pressure in the primary
section of the cylinder and in the primary circuit of the
The pressure created in the primary section of the
cylinder also acts against the back of the secondary
piston. This moves the secondary piston down its
bore to close off its compensating port and create
pressure in the secondary section of the master
cylinder and in the secondary circuit of the hydraulic
■ Pressure builds up simultaneously in both circuits
to apply the brakes at all four wheels.
Master cylinder construction
The dismantled parts of a master cylinder are shown in
Figure 24.8. This is a relatively simple design of a
The reservoir is a separate part, made of semitransparent material so that the fluid level is visible
without removing the cap. The reservoir is mounted on
top of the cylinder and there are seals between it and
the inlet ports of the cylinder.
The cylinder has a primary piston and a secondary
piston which are fitted with seals, and each has a return
spring. The pistons are retained in the cylinder by a
Basic tandem master cylinder
Dismantled tandem master cylinder
412 part four running gear
Each piston has two seals. The primary piston has an
L-type seal on its head and an O-ring on the opposite
end. The secondary piston has an L-type seal on its head
and another L-type seal in a groove in the opposite end.
The L-type seals are ring-shaped to fit into grooves in
the pistons. They have lips that press against the cylinder
bore and these are forced against the cylinder wall when
the brake is applied. The O-ring on the primary piston is
not subjected to pressure, but it is used to prevent fluid
from leaking from the rear of the cylinder.
Each seal is used for a different purpose and this
can be seen by considering the effects of faulty seals:
4. A faulty seal between the reservoir and the cylinder
will allow fluid to leak over the outside of the
Ports in the master cylinder
2. A faulty L-type seal on the rear of the secondary
piston will allow fluid under pressure to pass from
the primary section of the cylinder to the secondary
The master cylinder has two outlet ports to which the
brake lines are connected externally. One outlet is for
the primary circuit and the other is for the secondary
circuit. (These can be seen in Figure 24.8.)
The internal ports can be seen in Figure 24.9, which
is a sectional view of a master cylinder. There is a
compensating port and an inlet port between the
reservoir and the cylinder in both the primary section
of the cylinder and the secondary section of the
cylinder. These connect the reservoir to the cylinder
and are used to supply the cylinder with fluid.
The compensating ports are located just ahead of
the piston seals. The inlet ports are located behind the
piston seals and they supply fluid to the annulus area
of the pistons.
3. A faulty O-ring on the primary piston will allow
fluid to leak past the end of the piston and leak
from the rear of the cylinder.
■ Because of its compensating ports, this design of
master cylinder is referred to as a compensatingtype master cylinder.
1. A faulty seal on the head of a piston will cause loss
of pressure in a part of the cylinder.
Tandem master cylinder
1 reservoir cap, 2 reservoir seal, 3 reservoir, 4 sealing grommet, 5 secondary piston stop screw, 6 primary
compensating port, 7 primary inlet port, 8 secondary inlet port, 9 secondary compensating port, 10 primary piston, 11 primary
piston seals, 12 primary piston rod, 13 spring, 14 secondary piston seal, 15 primary seal for secondary piston, 16 seal retainer,
17 secondary piston spring, 18 secondary piston
chapter twenty-four brakes
Both Figures 24.8 and 24.9 are compensating-type
master cylinders. The internal compensating ports play
an important part in the operation of a master cylinder.
Master cylinder operation can be related to
Figure 24.9 as follows:
1. As the brake pedal is pressed, the primary piston is
moved along its bore. This closes off the primary
compensating port so that fluid pressure develops in
the primary circuit.
2. The fluid pressure created in front of the primary
piston forces against the rear of the secondary
piston, so that it also moves.
3. When the secondary piston moves, it covers the
secondary compensating port and pressure builds
up in the secondary circuit.
4. As the brake pedal continues to be pressed, both
pistons will move to displace fluid to their circuits
and apply the brakes.
■ While these have been described as separate
operations, they occur simultaneously to provide
equal pressure to both the primary and secondary
Action when the pedal is released
On the return stroke, the cylinder has an action known
as recuperation. This keeps the cylinder full of fluid
ready for the next brake application. This action for
one piston is shown in Figure 24.10 and it works like
1. When the brake pedal is released, the piston is
returned by its spring faster than the fluid can flow
back into the cylinder.
2. This creates a low pressure in front of the piston so
that, momentarily, the pressure in the reservoir is
higher than the pressure in the cylinder.
3. This causes a small amount of fluid to flow from
the reservoir, through the inlet port and past the seal
on the head of the piston to the front part of the
4. When the pedal is immediately pressed, the extra
fluid is trapped in front of the piston and the pedal
travel is reduced.
5. If the brake pedal is pumped, extra fluid will be
transferred to the front of the cylinder in this way.
Master cylinder piston on a recuperating
stroke – fluid flows past the primary cup
Each pedal stroke will reduce the travel, so that the
brakes can be held on with a very small pedal
6. When the pedal is released and the piston returns to
its normal position, fluid will flow back to the
reservoir through the compensating port so that all
pressure is relieved from the system.
Fail-safe feature of tandem cylinders
With a tandem cylinder, one section of the master
cylinder will still operate to provide emergency
braking if a hydraulic failure occurs in one circuit. This
is illustrated in the three diagrams in Figure 24.11.
1. Normal condition. In Figure 24.11(a), the cylinder
is full and the fluid is at the correct level in both
sides of the reservoir. There is no loss of fluid.
2. Leak in secondary. In Figure 24.11(b), fluid has
been lost from the secondary circuit. When the
brake pedal is pressed, there will be no resistance
from the secondary piston and it will bottom in the
The primary piston will travel further down the
cylinder bore, but will still develop pressure in
the primary circuit to provide emergency braking.
3. Leak in primary. In Figure 24.11(c), fluid has been
lost from the primary circuit, so the primary piston
will move along its bore until its piston rod contacts
the secondary piston.
The secondary piston will now be operated
mechanically by the primary piston, instead of
hydraulically so that the brakes operated by the
secondary circuit will still function.
414 part four running gear
The primary part of the cylinder has a compensating port and an inlet port, but there is a fast-fill valve
fitted between these ports and the reservoir.
The fast-fill valve is a combination valve with a
ball and a pressure seal. Depending on the pressure, it
allows fluid to flow from the large bore of the cylinder
to the reservoir, or from the reservoir into the large
The secondary part of the cylinder does not have
a compensating port. Instead, the centre valve in the
secondary piston performs the same function.
The valve closes when the secondary piston is moved
on the downstroke, so that pressure can build up ahead
of the piston. It opens when the piston is on the return
stroke to allow fluid to pass through the piston and
return to the reservoir.
■ The fast-fill arrangement enables the clearance
between the pads and the brake discs to be taken up
quickly when the pedal is pressed and this reduces
the pedal travel.
Master cylinder operation
Tandem master-cylinder and the effects of
(a) normal operation (b) loss of pressure in the secondary
circuit (c) loss of pressure in the primary circuit
■ Loss of fluid in any part of the system will increase
pedal travel and the brakes will operate on two
Centre-valve master cylinder
Centre-valve master cylinders do not have a
compensating port in the cylinder. Instead, they have a
centre valve in the piston which performs the same
Figure 24.12 shows the parts of a master cylinder
with a centre valve and Figure 24.13 shows the same
cylinder in cross-section. This master cylinder has a
centre valve in the secondary piston and a fast-fill
arrangement for the primary piston.
The bore of the cylinder is stepped, with a large
bore at the rear (open) end. The primary piston is also
stepped to suit the cylinder bore.
1. When the brake is applied, the primary piston is
moved in its bore and fluid is displaced by the
piston. The ball of the fast-fill valve is held on its
seat, so fluid is prevented from passing into the
2. Because of its larger bore, there will be more fluid
displaced by the rear part of the primary piston than
by the front. This extra fluid flows past the L-type
seal on the front of the primary piston and is added
to the fluid that is being displaced by the front of
3. This produces a high volume of fluid, which
quickly moves the brake pads to take up the
clearance between them and the discs. This is a
relatively high volume of fluid at a low pressure.
4. With continued movement of the piston, pressure
builds up in the system ahead of the piston to apply
5. There is also some pressure created in the large
bore behind the head of the piston. This pressure
opens the ball in the fast-fill valve so that fluid can
flow from the large bore back into the reservoir.
6. Once the fast-fill valve has opened, the cylinder
acts in the same way as a conventional master
cylinder, with all the pressure being created in the
small-bore section of the master cylinder.
chapter twenty-four brakes
Tandem master cylinder assembly with a fast-fill valve
1 cap, 2 seal, 3 reservoir, 4 retaining screw, 5 seal, 6 circlip, 7 fast-fill valve, 8 O-ring, 9 master cylinder body,
10 secondary spring, 11 secondary piston assembly, 12 piston stop pin, 13 retainer, 14 primary cup, 15 guide, 16 primary
piston, 17 O-ring, 18 end plug, 19 O-ring, 20 proportioning valve assembly, 21 O-ring, 22 sleeve, 23 differential switch FORD
fast fill valve
Sectional view of a tandem master cylinder with a fast-fill valve
416 part four running gear
When the brake pedal is released, the pistons will be
returned by their springs. Fluid will flow from the
primary section of the master cylinder back to the
reservoir through the compensating port, the inlet port
and the fast-fill valve.
From the secondary section, fluid will flow through
the centre valve and back to the reservoir through the
The centre valve performs the same function as a
compensating port. It is used in some master cylinders
fitted to vehicles with anti-lock braking systems. These
systems generally operate at higher pressures and have
more piston movement than standard brake systems.
Eliminating the compensating port from the cylinder
helps to prolong the life of the secondary piston seal.
Valves in the hydraulic system
Figure 24.14 shows, schematically, the valves that can
be used in hydraulic brake systems, although all these
valves are unlikely to be fitted to a system. They are
shown as separate valves in the diagram but, in an
actual system, may be designed as part of the master
cylinder or as combination valves.
Arrangement of a spool valve and switch to
operate a brake warning lamp
moved by the pressure in the other circuit. This will
raise the switch plunger to operate the differential
switch and light the brake-failure warning indicator.
The spool valve in the illustration has different
diameters at each end, but they are subjected to the
same hydraulic pressure. Nevertheless, the valve
remains in balance because the pressure on the large
end is opposed by the pressure on the small end, plus
the force of the spring.
Location of valves in a hydraulic brake
Pressure differential valve and switch
A pressure differential switch is shown in Figure
24.15. This is a plunger-type switch that is operated by
a spool valve connected between the two circuits of the
hydraulic system. With equal pressure in both circuits,
the valve is balanced and held in a central position.
Normally, the plunger of the switch rests in a
groove in the valve and the switch is off. However, if
there is a pressure loss in one circuit, the valve will be
Proportioning valves are used in braking systems to
regulate the hydraulic pressure to the rear brakes.
Generally, front brakes require a greater pressure than
the rear brakes, particularly during heavy braking.
Disc brakes require higher pressures than drum
brakes, so systems with disc brakes at the front and
drum brakes at the rear need a proportioning valve.
This is used to restrict the pressure to the rear brakes
and provide the correct proportion of braking to the
front and rear. Without this valve, the rear drum brakes
could lock before the front disc brakes were fully
The pressure to the drum brakes is kept to about 75%
of that of the disc brakes. However, if a failure occurs,
full pressure is provided by the proportioning valve.
■ The master cylinder in Figure 24.12 has a
proportioning-valve assembly. This valve has the
combined functions of proportioning and pressure
Principle of operation
Figure 24.16 is a diagram of a simple proportioning
valve. This is a form of pressure regulating valve,
chapter twenty-four brakes
from master cylinder
from master cylinder
the rear brake pressure will always be less than the
front. Any increase in pressure from the master
cylinder will affect both sides of the proportioning
valve so that the pressure difference will always
■ The crack pressure, or split pressure, is the
pressure at which the proportioning valve starts to
Arrangements of proportioning valves
Principle of a proportioning valve
sometimes referred to as a pressure-sensitive
regulating valve. It is shown as a separate valve, but
this type of valve is often a part of the master cylinder.
It is a stepped valve, operating in a stepped bore. The
arrangement is shown for a front–rear split system.
At lower pressures, the front and rear circuits
operate at the same pressure. At higher pressures, the
proportioning valve regulates the pressure in the rear
circuit so that it is less than that of the front circuit.
The following occurs when the brakes are applied:
1. Pressure from the primary side of the master
cylinder passes through the large part of the bore to
the front brakes.
2. At the same time, pressure from the secondary part
of the master cylinder passes through the smaller
part of the bore, via a passage in the valve, to the
rear brakes. To allow fluid to pass, the proportioning valve is held away from the poppet valve by the
3. As pressure in the system is increased, pressure
against the large end of the proportioning valve
overcomes the spring force and the proportioning
valve moves towards the poppet valve.
4. When a particular pressure is reached (crack
pressure) the proportioning valve will have moved
against the poppet valve to close off the passage to
the rear wheels.
5. With the passage closed, pressure will act against
the small end of the proportioning valve to oppose
the pressure on the large end. The passage will be
opened and closed as the proportioning valve
moves against or away from the poppet valve.
6. Because of the different areas on the ends of the
proportioning valve, a condition will exist where
There are various arrangements of proportioning
valves. The proportioning valve can be part of the
master cylinder (as in Figure 24.12), it can be a valve
(or valves) fitted to the master cylinder outlet, or it can
be a separate unit in the system.
Generally, a single unit is used where the system is
split between the front and the rear. For a diagonally
split system, the master cylinder can have two outlets
to the rear brakes with a valve fitted to each outlet.
Where a dual-proportioning valve is fitted into the
system away from the master cylinder, this will have
more than one valve as part of a unit.
Proportioning valve and brake failure
The type of proportioning valve in Figure 24.16 can
also operate if there is a brake failure that causes a
difference in pressure between the two circuits.
If there is loss of pressure in the rear brake circuit,
the proportioning valve will be moved against the
poppet valve to close off the rear circuit.
If the loss of pressure is in the front circuit, the
proportioning valve will be moved away from the
poppet valve by the spring so that full pressure reaches
the rear brakes. Movement of the proportioning valve
will also close off the front brake circuit.
These same actions can also be used to operate a
pressure differential switch to light the brake-failure
Load-sensing proportioning valve
There is a natural requirement that the braking force
applied to the front wheels should be greater than to
the rear. This is arranged by the size of brake
components and by the use of proportioning valves.
Load-sensing valves are used to reduce the braking
effect on the rear wheels and so increase the braking
effect on the front. This is a similar function to
pressure-sensitive proportioning valves.
With load-sensitive proportioning valves, there is a
mechanical connection between the valve, which is
418 part four running gear
mounted to the body of the vehicle, and the suspension
system. This ‘measures’ the displacement of the body
due to load and so the pressure is varied accordingly.
Load-sensing proportioning valves are used on
some larger vehicles, such as vans, which operate
loaded and unloaded. They are also used on some
small passenger vehicles where the load depends on
how many people are in the vehicle.
Disc brakes have wheel cylinders that are part of
the disc caliper. This straddles the brake disc and the
wheel cylinders clamp the brake pads against the disc.
Drum-brake wheel cylinders
There are two types of wheel cylinders used with drum
brakes: double piston (or double-acting) and single
Check valves were used in some drum-brake systems
to maintain a residual pressure in the system. The low
residual pressure was used to keep the wheel cylinder
Residual pressure is not needed with the design of
wheel cylinder cups now used in the wheel cylinders
of drum brakes. Residual pressure is not needed with
disc brakes and, in fact, is undesirable. Any pressure
remaining in the system when the brakes are released
would prevent the brake pads from releasing fully.
Figure 24.18 shows a double-piston cylinder in
sectional view. Each piston has a rubber cup which fits
into a groove in the inner end of the piston. A coil
spring between the pistons acts as a return spring and
this keeps the pistons apart. When the brakes are
applied, hydraulic pressure between the cups forces the
pistons outwards, and this forces the brake shoes
against the brake drum.
A rubber boot is fitted to each end of the cylinder to
exclude dirt and water. There is a bleeder valve which
is used when bleeding the brakes to remove air from
Figure 24.19 shows the parts of a dismantled wheel
cylinder. This is the type of wheel cylinder commonly
used for passenger vehicles.
Wheel-cylinder cups are installed with their sealing
lips pointing inwards. With the brakes released, and no
pressure in the system, the cups have sufficient
pressure against the cylinder to form a seal and hold
the brake fluid in the system. With the brakes applied
and pressure in the cylinder, the lips of the seals are
forced outwards against the wall of the cylinder to
increase the sealing action and hold the fluid under
A fluid-level warning device can be fitted to the
master cylinder reservoir. This consists of a float that
operates a sensor. When the float level drops below a
certain level, the sensor operates a warning light to
indicate this to the driver. One design is illustrated in
Level sensor in a brake master-cylinder
The wheel cylinders in both drum and disc brakes are
used to convert hydraulic pressure to a mechanical
force. The wheel cylinders for drum brakes are bolted
to the brake backing plate and are used to expand the
brake shoes against the brake drum.
Double-piston wheel cylinder for drum
chapter twenty-four brakes
Dismantled wheel cylinder with two pistons
Section of a disc and caliper showing the
cylinder, piston and pads
Disc brake caliper with its cylinder, piston
and seals FORD
The parts of a single-piston cylinder are shown in
Figure 24.20. It also has a ring-shaped cup which is
installed in a groove in the piston. This seals against
the cylinder and also against the piston.
Single-piston cylinders are used in pairs, with each
cylinder being used to operate one of the brake shoes.
Parts of a single-piston wheel cylinder
Disc brake cylinders
The cylinder of a disc brake is part of the caliper
assembly. The caliper straddles the disc and carries
the disc pads as well as the cylinder and piston
The parts of a caliper with a single piston are shown
in Figure 24.22. This is a typical single-piston caliper
for a passenger car. The cylinder is built into the caliper and the piston is much larger in diameter than those
used with drum brakes. The larger piston provides a
greater force which is needed for disc brakes.
The piston seal, a ring with a square cross-section,
is located in a groove machined in the cylinder. It fits
around the piston to provide a seal between the piston
and the cylinder. A boot fits into grooves in the
piston and the cylinder.
When the brakes are applied, fluid pressure behind
the piston forces it against the brake pad and the pad is
forced against the disc.
The pads have no return springs. The piston is
returned in its bore by the springy action of the piston
seal. This also prevents drag between the pad and the
disc. Figure 24.23 shows how the seal distorts when
the piston is moved in its cylinder, and how it relaxes to
withdraw the piston when the fluid pressure is relieved.
■ Some calipers have a single piston, some have two
pistons and the calipers of some high-performance
vehicles have four pistons.
The action of the disc brake piston seal
420 part four running gear
Hydraulic brake fluid
Hydraulic brake fluid is a special fluid designed to be
used only in hydraulic brake and clutch systems. Its
main requirement is to transmit force from the master
cylinder to the wheel cylinders, but it must also have
Brake fluids are glycol-based and must conform to
set standards, which include viscosity, boiling point
and compatibility with other brands of fluids and with
The brake booster operates whenever the brake
pedal is depressed, and the amount of assistance is
proportional to the pressure applied to the pedal.
Should the unit fail for any reason, the driver can still
apply the brakes, but a greater effort will be needed at
■ Vacuum-operated units of this type are given
various names, such as brake booster, vacuum
booster, vacuum servo and vacuum-assist unit.
Basic parts and operation
Care with brake fluid
Care must be taken with any spills of brake fluid
because brake fluid will damage paintwork. Spills
should be cleaned up immediately using plenty of water.
Care must also be taken to avoid contamination of
the fluid. It must not be mixed with any other liquid.
Containers used for brake fluid must be perfectly clean
and should not have previously been used for oil,
kerosene or any other mineral oil product.
Contaminated or incorrect fluid will cause the
rubber cups and hoses to swell and quickly become
■ Hydraulic brake fluid is covered in more detail in
Chapter 32: Fuels, fluids and lubricants.
The brake booster (Figure 24.24) assists the driver to
apply the brakes and so reduces the effort needed on
the brake pedal.
Passenger cars and light commercial vehicles with
petrol engines use a brake booster which is operated by
the partial vacuum produced in the engine’s intake
manifold. Vehicles with diesel engines cannot use
manifold vacuum and so they are fitted with an enginedriven vacuum pump.
The external parts of a basic brake booster are
identified in Figure 24.24 and the main internal parts in
Figure 24.25. Basically, the unit consists of two
vacuum chambers separated by a diaphragm. The
diaphragm is located between the brake-pedal pushrod
and the master-cylinder pushrod. A spring against the
diaphragm holds it in the released position.
There is a control valve in the rear chamber that is
operated by the brake-pedal pushrod. The valve can
admit either vacuum or atmospheric pressure to the
With the engine running and the brakes released,
there will be vacuum on both sides of the diaphragm
and the diaphragm spring will keep the diaphragm to
When the brakes are applied, the brake-pedal
pushrod operates the control valve. This shuts off
vacuum from the rear chamber and opens it to
Atmospheric pressure in the rear chamber moves
the diaphragm and the master-cylinder pushrod to the
left. This pushes against the piston in the master
cylinder to apply the brakes.
Arrangement of a brake booster and master
Principle of a brake (vacuum) booster with
the brakes being applied
chapter twenty-four brakes
When the brakes are being held (applied) the
diaphragm is kept stationary with vacuum on one side
and a partial vacuum on the other.
When the brake pedal is released, vacuum is
readmitted to the rear chamber and the return spring
pushes the diaphragm to its released position.
Operation of brake booster
The sectional views in Figures 24.26 to 24.28 show the
various parts of a brake booster unit in detail and also
how it operates. There are three conditions shown:
the released position, the applying position and the
Figure 24.26 shows the brakes in the released position
with vacuum on both sides of the diaphragm.
Atmospheric pressure, shown by the shading, is
The following relates to the released position:
1. The front chamber is connected by a hose to the
intake manifold so, whenever the engine is running,
there is vacuum in the front chamber.
2. The control valve is in the position where the
vacuum port V is open and the atmospheric port A
is closed. This allows vacuum to be admitted to the
rear chamber so that there is vacuum on both sides
of the diaphragm.
3. The diaphragm is held in the released position (to
the right) by the large diaphragm return spring.
With the valve in the position shown, the
vacuum port V is open, and both sides of the diaphragm are exposed to intake-manifold vacuum.
Atmospheric pressure, shown by the shading, is
blocked off at the atmospheric port A.
Figure 24.27 shows the brakes being applied with
atmospheric pressure on the rear of the diaphragm.
This is shown by the shading of the rear chamber.
The following relates to the applying position:
1. The brake pedal has been depressed and its pushrod
has transferred movement through to the master
2. Pressing the brake pedal also moves the plunger so
that the poppet C closes off the vacuum port V.
3. At the same time, the plunger B is moved away
from the poppet to open the atmospheric port A.
4. The atmospheric port A admits atmospheric
pressure to the rear chamber as show by the
5. The diaphragm is moved to the left to assist in
applying the brakes.
Figure 24.28 shows the brakes in the holding position.
There is intake-manifold vacuum on the front of the
diaphragm and a partial vacuum (pressure less than
atmospheric) on the rear. This is shown as light
shading of the rear chamber.
Brake booster – release position: the vacuum port V is open and the atmospheric port A is closed – both sides
of the diaphragm are under the influence of engine intake-manifold vacuum REPCO
422 part four running gear
Brake booster – applying position: when the brake pedal is depressed, the valve-operating rod attached to the
brake pedal moves the valve plunger forwards so that the valve poppet C closes off the vacuum port V –
the valve plunger B is moved away from the valve poppet to open the atmospheric port A
Brake booster – holding position: both the atmospheric port A and the vacuum port V are closed and the
diaphragm is held suspended with the brakes applied – partial vacuum is retained in the rear chamber to hold
The following relates to the holding position:
1. With the brakes applied, pressure in the master
cylinder pushes back against its pushrod and the
valve plunger to close the atmospheric port A.
2. This retains a partial vacuum in the rear chamber
with the diaphragm held suspended.
3. The diaphragm will remain in an applied position. The
vacuum and the spring in the front chamber will be
balanced by the partial vacuum in the rear chamber.
4. An increase in pedal pressure will move the plunger
off its seat to admit atmospheric pressure to the rear
chamber and apply the brakes harder. A decrease in
pedal pressure will have the opposite effect.
chapter twenty-four brakes
5. When the pedal is released, the plunger and poppet
will move back to the released position. This will
open the vacuum port V to admit vacuum to the
rear chamber and the return spring will move
the diaphragm back to the release position.
■ The term vacuum is used to indicate a negative
pressure, which is a pressure below atmospheric.
A true vacuum would be a condition where there is
no air and no pressure.
The booster that has been discussed has a single
diaphragm and one atmospheric chamber. Some larger
vehicles have dual diaphragms and two atmospheric
chambers. These operate in a similar manner to that
described for a single diaphragm. The main difference
is that, with atmospheric pressure acting against two
diaphragms, there will be twice the assistance provided
to apply the brakes.
There are two main parts to a drum brake: the cast iron
brake drum and the brake-shoe assembly. The latter
includes the backing plate with the brake shoes, wheel
cylinders and other associated parts.
The dismantled parts of a drum-brake assembly for
a passenger car are shown in Figure 24.29, and a drumbrake assembly for a light commercial vehicle is
shown in Figure 24.30.
The brake drum provides a cast-iron braking surface
against which the brake linings operate. The brake
shoes are expanded inside the drum and so the drum
must be capable of withstanding the force applied by
the brake shoes without distorting.
The brake drum must be capable of absorbing the
heat produced by friction between the shoes and the
drum. It must also dissipate this heat to prevent
excessively high temperatures developing in the brake
Cast iron is used for brake drums because it has
properties which make it suitable for this purpose.
Some brake drums are made entirely of cast iron but
other brake drums are made with a cast iron braking
surface and a steel web.
The backing plate for the front brakes is mounted to
the steering knuckle, and the backing plate for the rear
brakes is mounted to the axle flange. The backing plate
carries all the stationary brake parts, which include the
wheel cylinders, brake shoes, return springs, retaining
springs, anchor and adjuster.
The backing plate is a steel pressing which has its
outer edge flanged to fit over the edge of the drum. It
not only acts as a support for the brake shoes and
associated parts, but also acts as a shield to exclude
Dismantled rear drum-brake assembly with parking-brake parts
424 part four running gear
Drum-brake assembly of the duo-servo design
1 rear-axle housing, 2 hydraulic pipe, 3 backing plate, 4 anchor, 5 return spring, 6 brake drum, 7 axle flange,
8 adjuster, 9 brake shoe, 10 handbrake cable TOYOTA
Brake shoes and linings
Two brake shoes are used and these are shaped to fit
the contour of the brake drum. A shoe consists of a
web and a flange. The web is provided to stiffen the
flange and prevent the shoe from distorting when it is
expanded. The flange has a lining of friction material
bonded to it. The lining is a compound consisting of
various fibres, synthetic resins, friction materials and
Linings are designed to withstand the high temperatures that are generated during braking. To achieve
this, they must be able to transfer some of the heat to
the brake shoes. Some of the fibres used in brake
linings are insulators of heat and, to offset this, soft
particles of metal, such as zinc, can be included in the
lining material. The metal particles are good
conductors of heat and so help to transfer heat from the
surface of the lining to the shoe.
Springs are fitted between the shoes, or between the
shoes and the backing plate. These are used to locate
the shoes on the backing plate and to return the shoes
to their normal position when the brakes are released.
Retaining springs and clips are used to hold the
brake shoes against the backing plate. Other springs
are used to locate the shoes in position or to hold the
Anchors are used to locate the ends of the shoes by
providing an abutment against which the shoes can
rest. Anchor pins are used to hold the ends of some
■ Adjusters are used to provide the working
clearance between the brake shoes and the drums.
These are covered in the following chapter.
The wheel cylinders are bolted to the backing plates.
Some backing plates carry one cylinder, others have
two cylinders. One end of each brake shoe abuts a
piston and the shoes are expanded by the pistons when
the brakes are applied. As previously indicated, some
wheel cylinders have two pistons and others have only
Various arrangements are used to mount the brake
shoes to the backing plate, although there are three
general designs of brake assemblies that have been
1. leading and trailing shoes
3. two-leading shoes.
Leading and trailing shoe brakes
Figure 24.31 illustrates a basic brake assembly with
leading and trailing shoes. The shoes rest against an
chapter twenty-four brakes
plate in a way that uses the wedging effect to
■ This wedging effect is referred to as a selfenergising effect or a servo effect.
Basic brake-shoe assembly with a leading
and a trailing shoe
anchor at their lower end (the heel of the shoe) and are
expanded at their upper end (toe) by means of a
double-acting wheel cylinder. They are held in position
by the return springs. (The brake assembly in Figure
24.29 also has leading and trailing shoes.)
The hydraulic action of the wheel cylinder, when
the brakes are applied, pivots both shoes about the
anchor to force their upper ends against the brake
drum. The direction of rotation of the drum, shown by
the arrow, assists the movement of the leading shoe.
Once the brake lining comes into contact with the
drum, rotation gives it a wedging action against
the drum. The lower end of the shoe can also move
on the anchor so that there is full contact between the
lining and the drum.
Rotation has the opposite effect on the trailing
brake shoe, which receives no assistance from drum
rotation. However, when the vehicle is moving in
reverse, the drum will rotate in the opposite direction
and the rear shoe will have the wedging action.
Generally, with this type of assembly, the increase
in braking gained by the leading shoe as the result of
drum rotation is offset by the loss suffered by the rear
shoe. This can result in greater wear of the front shoe
than the rear shoe.
The leading-shoe lining is at times made longer
than that of the trailing shoe to provide a greater area
of lining in order to even up the rate of wear of the two
All brake shoe assemblies have this type of
wedging action to a greater or lesser extent, but certain
designs have the brake shoes mounted on the backing
Duo-servo brakes (Figure 24.32) have a self-energising
or servo effect on both brake shoes. The shoes are
linked together at their lower ends and are allowed to
float – the lower ends are not anchored to the backing
For self-energising assemblies, the front shoe is
known as the primary shoe, and the rear shoe as the
On applying the brake, the primary shoe comes in
contact with the drum and is rotated slightly with the
drum to transfer movement through the linkage to
the secondary shoe. The secondary shoe therefore
receives force from both ends, that is, from the
expansion of the wheel cylinder at the upper end and
the transfer of movement from the primary shoe at the
lower end. The combined effect causes the secondary
shoe to be forced against the drum even harder than the
■ Because they operate differently, primary and
secondary shoes can have different lengths of
linings, or different types of linings.
Brake assembly of the duo-servo type
This design has two single-acting wheel cylinders
(Figure 24.33) and these produce a wedging action on
both the front and the rear shoes.
426 part four running gear
(a) Fixed caliper
Two-leading-shoe brake assembly
1 brake shoe, 2 adjuster, 3 cylinder, 4 return
spring, 5 retainer, 6 backing plate TOYOTA
Each cylinder forces one end of its shoe outwards,
and a self-energising effect is imparted to each shoe
due to drum rotation as previously explained. The
other end of the shoe is located against the back of the
wheel cylinder which acts as an anchor. Therefore,
both shoes provide equal braking, with a force greater
than that which could be applied by normal means.
Some vehicles, with drum brakes at both the front
and rear, have two-leading-shoe brakes at the front
wheels and leading and trailing type brakes at the
rear wheels. This provides the higher percentage of
braking required for the front wheels of the vehicle.
Simple disc-brake assemblies are illustrated in
Figure 24.34. They consist of two main parts: the disc
(also called the rotor) and the caliper assembly. The
disc rotates with the wheel hub while the caliper,
which straddles the disc, is held stationary. The caliper
assembly includes the hydraulic cylinder, piston and
the brake pads. Calipers for rear brakes also have a
mechanism for applying the parking brake.
The diagrams show the principle of disc brake
operation. With the brake in the released position, the
pads are slightly clear of the disc which rotates
between them. When the brake is applied, pressure
from the master cylinder forces the pistons against the
pads, which are then forced against the disc. This
produces a clamping action, which slows or stops the
(b) Floating caliper
Disc brake arrangements for fixed and
When the brake is released, the pistons retract
slightly to allow the pads to move away from the disc.
The pads have no return springs, but the pistons are
returned slightly in their bores by the resilience of the
piston seals. A small runout of the disc moves the pads
away from the disc surface to provide clearance and
Two different designs of calipers are shown: a fixed
caliper and a floating caliper. The fixed caliper has a
cylinder and piston on each side of the disc, while the
floating caliper has a cylinder and piston on only one
side of the disc.
Features of disc brakes
Some of the features of disc brakes are:
1. The surface of the disc is exposed to the
atmosphere, and the heat can dissipate.
2. Higher pressures are required in the system to
produce a large force against the small friction pad.
3. The disc is exposed to water and dirt, but these are
easily shed from the brake assembly because of its
chapter twenty-four brakes
4. A splash guard, or backing plate is fitted to protect
the disc from road dirt.
5. Balance of the disc is easy to achieve during manufacture because of its comparatively simple shape.
6. Braking is uniform under nearly all conditions, with
little possibility of brake grab or brake fade.
There are two designs of brake discs: solid discs and
ventilated discs. Ventilated discs are often used on the
front brakes and solid discs are used on the rear brakes.
Brake discs are made of cast iron, with a ground
surface on each side against which the pads are
applied. The disc is shaped to fit over the wheel hub
and has drilled holes to fit the wheel studs.
Figure 24.35 illustrates a ventilated disc. This is of
hollow construction, consisting of two flanges separated by fins. The rotating disc acts as a form of air
pump to maintain a flow of air through the disc, and so
removes heat generated during braking. (See also
■ Ventilated discs are used with front brakes because
they are applied with greater force than the rear
brakes and so generate more heat.
Disc brake assembly with a fixed caliper
1 inner caliper housing, 2 outer housing,
3 bleeder valve, 4 pad locating pins, 5 steady springs, 6 piston
seal, 7 boot, 8 retainer, 9 piston, 10 pad, 11 disc
applied to the pistons and they move the pads against
the disc with equal force. Only the pistons move – the
caliper is fixed to the steering knuckle.
Figure 24.37 shows a fixed caliper with four
pistons. This type of brake is used on some highperformance vehicles.
Ventilated disc and caliper
Front brake with a ventilated disc, a fixed
caliper and four pistons MAZDA
A fixed-caliper assembly and a solid disc are illustrated in Figure 24.36. In this design, a two-piece
caliper is used, with the two parts being bolted
together. The caliper has two cylinders, one on each
side of the disc, and so each piston operates its own
When the brakes are applied, hydraulic pressure is
The dismantled parts of a floating-caliper assembly are
shown in Figure 24.38. The main parts of the assembly
are the caliper housing, the anchor plate, the piston and
the pads. This type of caliper is made as a single part
and has only one cylinder, which is built into the inner
side of the caliper. The anchor plate is an iron casting
and the caliper housing is a light alloy casting.
428 part four running gear
Disc brake caliper assembly with a floating caliper
The caliper is mounted on the anchor plate which is
bolted to the steering knuckle. The caliper is not
rigidly attached to the anchor plate – a machined
surface on the caliper rests on a corresponding
machined surface on the anchor plate.
Two guide pins, which are bolted to the caliper,
pass through holes in the anchor plate. The guide pins
position the caliper in relation to the anchor plate but
allow it to slide sideways (float) in operation.
Floating caliper operation
The operation of a floating caliper can be seen in
1. In Figure 24.39(a), with the brakes released, there is a
small running clearance between the pads and the disc.
2. In Figure 24.39(b), hydraulic pressure in the
cylinder has forced the piston against the inner pad,
and the inner pad against the disc.
(a) Released position
(b) Applied position
Floating-caliper operation (piston seal action is shown in the insert)
1 piston, 2 cylinder, 3 caliper body, 4 outer pad, 5 disc, 6 inner pad, 7 piston seal, 8 boot, 9 fluid inlet
chapter twenty-four brakes
3. Hydraulic pressure acts against the back of the
cylinder as well as against the piston, and this
causes the caliper housing to slide inwards.
4. This movement is transferred through the caliper to
the outer pad, which is also forced against the disc.
Both pads are applied with equal force.
This design is often used with front brakes. Its
operation is similar to that of a single piston caliper,
except that two pistons provide a greater braking force.
The illustration has the caliper cut away so that the
pistons and brake pads can be seen.
When the brake pedal is released and hydraulic
pressure removed, the piston is withdrawn a little by
the action of the piston seal. This removes pressure
from the pads and provides running clearance between
the pads and the disc.
The action of the piston seals is also shown in the
illustration. In the released position, the seal is relaxed,
but it flexes when the brakes are applied. When the
hydraulic pressure is relieved, the seal returns to its
normal shape, taking the piston with it. This provides
the small clearance that is needed between the brake
pad and the disc.
Dual-piston brake caliper
Self-adjustment of pads
The brake assembly in Figure 24.40 has a floating
caliper with two cylinders and two pistons. It also has
a ventilated disc.
Disc brakes are self-adjusting. As the pads wear, the
pistons will gradually move further from their bore to
A disc pad consists of a steel backing plate with
friction material bonded to its surface (Figure 24.41).
The pad is positioned by guide lugs that fit into slots
in the caliper or in the anchor plate. Anti-rattle clips
are fitted to the lugs to prevent the pad from rattling
in the slots when the brakes are released. A steel shim
is often fitted between the back of the inner pad and
the piston. The parts shown are typical, but various
shapes and sizes of pads are used in other installations. Different types of anti-rattle springs are also
Cut-away view of a floating caliper assembly
with two pistons and a ventilated disc SUBARU
Disc brake pads
430 part four running gear
maintain the small clearance between the pad and
the disc. The pistons will be retracted by the action of the
seals each time the brakes are released, but only to a
position which gives the normal pad clearance.
Also, with floating calipers, the caliper housing is
being centred each time the brakes are applied. In this
way, disc brakes adjust themselves. There is no service
Most parking brakes are operated by a hand lever that
is floor-mounted and located beside the driver.
However, there are also parking-brake levers that are
dash-mounted and parking pedals that are foot
operated. There are many different arrangements.
Figure 24.42 illustrates the general layout of a
floor-mounted parking brake (handbrake). This consists of the parking-brake lever with cables connecting
it to the rear brake assemblies. The lever is connected
to a yoke which carries the cables. This acts as an
equaliser so that equal pull is applied to both cables.
The front end of the cable is exposed, but the rear ends
are enclosed by an outer cable. Brackets and guides are
provided to locate the cable in relation to the
bodywork and suspension.
The parking-brake lever includes a ratchet mechanism
which consists of a toothed quadrant on the mounting
bracket and a pawl on the lever. When the parking
brake is applied, the spring-loaded pawl engages with
the teeth in the quadrant and holds the brake on. The
brake is released by pressing the button to disengage
Parts of a parking-brake assembly
Some vehicles have a dash-mounted parking brake.
One design that is used in some light-commercial
vehicles is shown in Figure 24.43. This has a T-shaped
handle attached to a rod. When the handle is pulled
outwards to apply the brake, pawls (located at position
A) drop into ratchet teeth cut into the rod to hold the
brake in the applied position.
The rod operates a lever that is attached to a front
cable. The rear end of this cable is connected to an
equaliser on the rear cables which transmit movement
of the handle to the rear brakes.
The brake is released by rotating the handle. This
turns the teeth away from the pawls so that the rod can
slide inwards to the released position.
Dash-mounted parking brake
A foot-pedal operated parking brake is used in some
vehicles, although these are not common. This is a
small pedal that operates the parking brake cable in the
same way as a hand lever. It is a separate pedal that
has no connection to the normal foot-brake pedal.
chapter twenty-four brakes
When the pedal is pushed to apply the parking brake,
a pawl and ratchet device holds the brake applied. The
parking brake is released by again pressing the pedal.
Drum parking brake
With drum brakes, the rear end of the parking-brake
cable is attached to a lever at the brake assembly
One end of the lever is pivoted on a pin at the top of
the rear shoe, and the other end is in the form of a hook
to which the brake cable is attached. A pushrod, which
sometimes incorporates the brake adjuster, is fitted
between the lever and the front shoe.
When the lower end of the lever is pulled by the
cable, the pushrod pushes the front shoe against the
brake drum. The reaction of the lever against the rear
shoe also pushes the rear shoe against the brake drum.
■ This brake assembly is of the leading and trailing
shoe design and is a typical assembly for a rear
Parking brake for disc brakes
There are two designs of parking brakes for disc
brakes: drum parking brakes and disc parking brakes.
One design has a combination of disc and drum,
with the drum being used for a separate parking brake.
In the other design, the parking brake uses the brake
Disc with drum parking brake
Figure 24.45 shows a disc brake with a separate brakedrum assembly for the parking brake. A combined
drum and disc is used. The disc brake operates in the
usual way on the disc section and the parking brake
operates inside the drum.
The parking-brake assembly is smaller than a
normal drum brake. It is operated by the brake cable
Rear drum-brake assembly showing the
Disc brake with a separate drum-type parking brake