Brakes 407 Basic brake system 408 Hydraulic principles 408 Brake hydraulic systems 410 Master cylinders 410 Compensatingtype master cylinder 413 Centrevalve master cylinder 414 Valves in the hydraulic system 416 Wheel cylinders 418 Hydraulic brake fluid 420 Brake booster 420 Drumbrake assemblies 423 Brakeshoe assemblies 424 Discbrake assemblies 426 Parking brakes 430 Technical terms 432 Review questions 432
Trang 1Compensating-type master cylinder
Centre-valve master cylinder
Valves in the hydraulic system
Trang 2A 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.
Operation
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.
Hydraulic principles
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 principle.
figure 24.1 Arrangement of diagonally-split brake system FORD
left front caliper
brake lines
secondary primary
Trang 3Liquids will not compress
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.
■ 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
brakes.
■ When work is done on hydraulic brakes, air can
enter the system and the brakes have to be bled to
remove the air.
Force and pressure
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.
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
figure 24.2 Force applied to a liquid transmits pressure
liquid
piston
cylinder force
figure 24.3 Force applied to the piston compresses the
air but not the liquid
figure 24.4 Force is transmitted by hydraulic means from
one cylinder to another – equal cylinders, equal force
piston
force force
figure 24.5 Basic principle of a hydraulic brake system
Trang 4force 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 mm 2 ,
in (B) it is 20 mm 2
and in (C) it is 160 mm 2
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
(A).
■ 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.
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.
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.
Diagonally-split systems
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 circuit.
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.
Master cylinders
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 used.
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
figure 24.6 Basic master cylinder connected to a wheel
cylinder
check valve
wheel cylinder
Trang 5compensating 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 hydraulic system.
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 system.
■ 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 tandem cylinder.
The reservoir is a separate part, made of transparent 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.
semi-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 snap ring.
figure 24.7 Basic tandem master cylinder
primary outlet
compensating port
inlet port
figure 24.8 Dismantled tandem master cylinder HYUNDAI
L-type seals
O-ring
Trang 6Piston seals
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:
1 A faulty seal on the head of a piston will cause loss
of pressure in a part of the 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
section.
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.
4 A faulty seal between the reservoir and the cylinder will allow fluid to leak over the outside of the cylinder.
Ports in the master cylinder
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.
■ Because of its compensating ports, this design of master cylinder is referred to as a compensating- type master cylinder.
figure 24.9 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
Trang 7master cylinder
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
circuits.
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
this:
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
cylinder.
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.
Each pedal stroke will reduce the travel, so that the brakes can be held on with a very small pedal movement.
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 cylinder bore.
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.
figure 24.10 Master cylinder piston on a recuperating
stroke – fluid flows past the primary cup
Trang 8■ Loss of fluid in any part of the system will increase
pedal travel and the brakes will operate on two
wheels only.
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
function.
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.
The primary part of the cylinder has a ing port and an inlet port, but there is a fast-fill valve fitted between these ports and the reservoir.
compensat-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 bore.
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
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 reservoir.
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 the piston.
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 the brakes.
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.
figure 24.11 Tandem master-cylinder and the effects of
leaks (a) normal operation (b) loss of pressure in the secondary
circuit (c) loss of pressure in the primary circuit
Trang 9figure 24.12 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
12 11 10
13 14 15 16
17
23 20
22 21 19 18
9 8 7 6 5 4 3 2 1
figure 24.13 Sectional view of a tandem master cylinder with a fast-fill valve FORD
reservoir sealing grommets
secondary piston
return spring
secondary seals L-type
primary seal L-type
O-ring
caged spring
split line primary
piston O-ring fast fill valve circlip reservoir
clip reservoir cap
reservoir cap seal
Trang 10Return stroke
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
inlet port.
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.
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.
Proportioning valve
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 applied.
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 differential.
figure 24.15 Arrangement of a spool valve and switch to
operate a brake warning lamp
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
Trang 11sometimes 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
proportion-ing valve is held away from the poppet valve by the
spring.
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
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 exist.
■ The crack pressure, or split pressure, is the pressure at which the proportioning valve starts to operate.
Arrangements of proportioning valves
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 indicator lamp.
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
figure 24.16 Principle of a proportioning valve
from master cylinder
to front brakes
from master cylinder (primary) proportioning valve
Trang 12mounted 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.
Check valve
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
cups expanded.
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.
Fluid-level warning
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
Figure 24.17.
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 piston.
Double-piston cylinders
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 the system.
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 pressure.
figure 24.17 Level sensor in a brake master-cylinder
Wheel cylinders
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.
Trang 13Single-piston cylinders
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.
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.
figure 24.19 Dismantled wheel cylinder with two pistons
HOLDEN LTD
figure 24.20 Parts of a single-piston wheel cylinder
figure 24.21 Section of a disc and caliper showing the
cylinder, piston and pads
figure 24.22 Disc brake caliper with its cylinder, piston
and seals FORD
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
(Figure 24.21).
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
cali-per 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. figure 24.23 The action of the disc brake piston seal
Trang 14Hydraulic 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
other properties.
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
brake-system parts.
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
unserviceable.
■ Hydraulic brake fluid is covered in more detail in
Chapter 32: Fuels, fluids and lubricants.
Brake booster
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
engine-driven vacuum pump.
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 the pedal.
■ 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
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 rear chamber.
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 the right.
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.
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.
figure 24.24 Arrangement of a brake booster and master
cylinder
figure 24.25 Principle of a brake (vacuum) booster with
the brakes being applied
diaphragm
rear chamber (atmospheric)
pedal pushrod
diaphragm spring
master cylinder pushrod
front chamber (vacuum)