Brake service 435 Maintenance and inspection 436 Brake booster 438 Hydraulic system – service points 439 Bleeding brakes 441 Master cylinder service 442 Drumbrake service 443 Drumbrake adjustments 446 Selfadjusting drum brakes 447 Discbrake service 448 Parking brakes 451 Brake tools 453 Fault diagnosis 454 Technical terms 455 Review questions 455
Trang 2Brake service involves maintenance and repair of drum
brakes, disc brakes, the parking brake, or the hydraulic
system.
Servicing the hydraulic system includes checking
the level of the fluid and changing the fluid, and also
renewing cups and seals if there is a leak in the system.
The brake linings of drum brakes and the pads of disc
brakes gradually wear away and have to be replaced
when they become excessively worn.
Some brakes require adjustments to compensate for
wear, although most braking systems have automatic
adjustments.
This chapter covers standard brakes Antilock
brak-ing systems (ABS) are covered separately in the
following chapter.
Maintenance and inspection
The following paragraphs outline the maintenance and
inspection requirements of the various parts of brake
systems.
Brake fluid level
The level of the hydraulic fluid in the master cylinder
reservoir should be checked regularly Reservoirs are
made of transparent material and have maximum and
minimum fluid level marks This enables the fluid
level to be checked without removing the reservoir cap
(Figure 25.1).
The fluid in the reservoir should be maintained at
the maximum level and normally should not require
topping up If frequent topping-up is necessary, the
system should be checked for leaks.
The cap and seal, or the cover and seal, should be
refitted to the reservoir correctly so that the fluid will
not leak or be exposed to the atmosphere.
■ Brake fluid that is exposed to the air will absorb
moisture from the air and this will lower its boiling
point.
Fluid leaks and spills
If fluid is being lost from the reservoir, the system should be checked for signs of leaks Locations of possible fluid leaks are the master cylinder, the wheel cylinders, the brake hoses, the brake lines and connections.
Care should be taken when handling and using brake fluid as it will cause damage if spilt on paintwork Should this occur, the area should be immediately washed with cold water.
Brake fluid change
Brake fluid gradually deteriorates and becomes contaminated It also absorbs moisture and this reduces its boiling point.
Brake fluid should be changed regularly – the usual recommendation is every two years The system is bled, as described later, to remove all the old fluid Fluid is bled from each wheel until new, clean fluid flows from the bleeder.
Brake pedal checks
Checks can be made of the brake pedal as shown in Figure 25.2 These are the free height, free play, pedal travel and pedal reserve.
1 Free height The free height, or distance of the pedal above the floor is a specified dimension (Figure 25.2(a)) This can be adjusted by altering the length of the pushrod at the brake pedal.
2 Free play The pedal should also have a small amount of free play (3 to 6 mm) in the released
figure 25.1 Fluid level in the master cylinder reservoir
figure 25.2 Brake pedal
1 free height, 2 pedal travel, 3 pedal reserve,
4 floor, 5 free play, 6 stop-lamp switch, 7 pushrod adjustment
Trang 3position This is the amount of movement at the
pedal before any resistance is felt If free play is
insufficient, pressure will build up in the system
and cause the brakes to be slightly applied On the
other hand, if the free play is excessive, this could
reduce the effective pedal travel.
3 Pedal travel This is the distance that the pedal
travels when the brakes are applied (Figure 25.2(b)).
4 Pedal reserve The depressed pedal height, or
distance from the floor, is known as the pedal
reserve.
Pedal reserve
Pedal reserve is related to pedal travel If the pedal has
normal travel, then it will be well clear of the floor
when the brakes are applied and there will be
ade-quate pedal reserve However, if there is excess pedal
travel for some reason, then there will be a low pedal
reserve.
There are different causes for low pedal reserve as
indicated below.
■ Pedal reserve must be checked with the parking
brake off so that the brake shoes or pads are in the
released position.
Low pedal reserve – firm pedal
Excessive pedal travel, but with a firm pedal, could
mean that there is too much clearance between the
brake shoes and the drum The brakes could need
adjusting, or the self-adjustment mechanism might not
be working Worn brake linings or pads would also be
a likely cause.
In the hydraulic system, low pedal reserve could be
the result of failure of one of the circuits of the
split-brake system.
Low pedal reserve – soft pedal
Excessive pedal travel, but with a soft or spongy pedal,
could be caused by air in the hydraulic system, or by a
shortage of brake fluid.
It could also be the result of other hydraulic faults
that prevent pressure from being built up, such as a
defective master cylinder If the pedal moves slowly
down, under light to medium pressure, the fault is
likely to be the seals in the master cylinder.
■ It is unsafe to operate a vehicle with low pedal
reserve and the system should be checked to find
the cause.
Stop-light switch
The operation of the stop-light switch should also be checked The switch is operated by brake pedal move- ment, being ‘off’ when the brake pedal is in the released position, and switched ‘on’ as soon as the brake pedal is moved.
The switch can be adjusted by altering its position
in relation to the pedal This can also be used to adjust pedal free play.
Brake lines and hoses
The brake lines should be inspected for damage and the hoses for deterioration and signs of leaks (Figure 25.3).
Hoses should be installed so that they are free of twists and sharp bends They should be positioned so that they will not rub against parts of the suspension or
be fouled by the tyre when the front wheel is turned The nuts of brake line connections should be tight The tubing should be correctly secured and free of kinks and dents.
figure 25.3 Inspection of hydraulic brake hose and tubing
TOYOTA
Brake adjustments
Disc brakes do not require adjustment of the pads because they are self-adjusting The pistons gradually move out of their cylinders as the pads wear to maintain a small clearance between the pads and the discs This was described in relation to floating-caliper operation in the previous chapter.
Trang 4Most drum brakes have self-adjusting mechanisms,
but there are some that have manual adjustments These
will require periodic adjustment to compensate for
brake-lining wear (Brake adjustments are covered later.)
Lining and pad wear
Use of the brakes gradually wears away the friction
material of the brake linings and the brake pads until a
point is reached where they have to be replaced.
The thickness of brake pads can usually be checked
visually without removing them from the caliper
assembly Calipers often have an inspection hole
through which the edges of the pads can be seen
(Figure 25.4).
Some brake pads have a metal spring which acts as
a wear indicator (Figure 25.5) When the pad has worn
so that there is only about 1.5 mm of facing left, the
spring makes contact with the disc when the brakes are
applied This produces a metallic sound that warns the
driver that the pad is near its wear limit.
With drum brakes, the wheels and brake drums
have to be removed to check the thickness of the brake
linings.
Parking brake
The parking-brake lever, when fully applied, should
only move about half its possible travel as shown in
Figure 25.6.
As the lever is moved to the applied position, the pawl will click over the ratchet teeth The maximum number of clicks is often specified, and this is used as
a gauge of lever movement Excessive movement indicates the need for adjustment.
Brake booster
The operation of the brake booster can be checked with the engine running and with it stopped (Figure 25.7) This will show whether the unit is working and providing assistance The booster can also be checked for leaks.
Check of operation
Operation of the unit can be checked as follows:
1 With the engine off, depress the brake pedal several times to ensure that no residual vacuum exists in the unit.
2 Hold the pedal depressed and start the engine The booster should now help with brake application.
figure 25.4 Disc brake assembly – the caliper has an
inspection hole
figure 25.5 Operation of a pad wear indicator HYUNDAI
figure 25.6 Parking-brake (handbrake) travel is checked
by the number of clicks of the pawl over the ratchet teeth TOYOTA
Trang 5The pedal should feel easier to push and there
should be a noticeable increase in pedal travel.
Check for leaks
The unit can be checked to ensure that it is airtight and
holds vacuum If there are no leaks, the booster should
provide two or three assisted applications of the brakes
after the engine is switched off:
1 Switch off the engine and depress the brake pedal.
Note the ease of application and the distance the
pedal travels The pedal should operate normally.
2 After two or three applications, the vacuum in the
unit will be lost The pedal will have greater
resis-tance and slightly less travel than before, indicating
that the unit is no longer operating This will occur
if the unit is normal.
If the booster does not operate after the engine is
switched off, a leak is indicated The hose to the intake
manifold should be checked for tightness The booster
check valve at the end of the hose should also be
checked for operation.
■ This valve opens when the engine is running and
closes when the engine is stopped to retain vacuum
in the booster.
Brake booster repairs
Some boosters can be dismantled for repair, but others
are not normally repairable and have to be replaced if
faulty.
Boosters that are repairable are designed so that the two chambers can be separated and access gained to the diaphragm and valves Special holding tools are used during dismantling and reassembly.
Hydraulic system – service points
Illustrations in the previous chapter show the construction of master cylinders and wheel cylinders, and these enable the arrangement of the various parts
to be seen (also see Figure 25.12).
Dismantling and reassembling are not involved operations, but there are points that should be observed.
When dismantling, it is good practice to pay particular attention to the order in which parts are removed so that there are no doubts when the time comes for reassembly The location of each seal or other replaceable part should be identified as it is being removed.
■ Seals should be checked to see how they are fitted The sealing lips will not seal unless they are pointing in the correct direction.
Cleaning and inspection
All dismantled parts should be washed in methylated spirits or brake fluid and inspected for wear and damage All rubber seals should be renewed.
Aluminium alloy cylinder bores should not be honed If pitting is excessive, the cylinder should
be replaced, although some cylinders can be restored
by fitting a sleeve.
Cast iron cylinder bores that are pitted or scored can be honed with a brake cylinder hone If more than 0.1 mm of metal is removed then oversize cups have to
be fitted.
Extreme care must be exercised when dismantling hydraulic brake parts Mineral oil, grease, petrol or kerosene must not come into contact with any hydraulic parts The rubber seals will quickly deterior- ate after contact with any mineral oil product.
Reassembling components
During reassembly of the master cylinder and wheel cylinders, cups and lip-type seals must be installed with the sealing lips pointing in the right direction, that
is, with the lips towards the pressure area of the cylinder so that they will be expanded against the bore when fluid pressure is applied.
figure 25.7 Checking the operation of a brake booster –
the pedal travel is checked with the engine both running and stopped TOYOTA
Trang 6The cups and pistons can be coated with brake fluid
before being installed, although a special rubber grease
is also used Rubber grease is used on the parts of
wheel cylinders and calipers as shown in Figure 25.8.
With master cylinders, the cups, pistons and the
open end of the bore should be coated with rubber
grease.
Brake fluid
Only clean, new fluid should be used in the hydraulic
brake system Fluid that has been used to bleed the
brakes should not be reused.
Brake fluids are specified as Dot 3 and Dot 4 and
this will be marked on the fluid container This
specifi-cation relates to the boiling point of the fluid and only
fluid meeting these specifications should be used.
The cover should be kept on the master cylinder
and the fluid container should always be capped to
prevent the fluid from absorbing moisture from the
atmosphere.
Hydraulic brake lines
Hydraulic connections must be tightened correctly to
avoid leaks and a flare-nut spanner is the most suitable
tool to use It is designed specially for the nuts on the
ends of the tubing The spanner is ring-shaped, but
slotted so that it will fit over the tubing and then onto the nut, as shown in Figure 25.9 Some flare nuts are quite small (6 or 8 mm across the flats) and are difficult to loosen or tighten with a normal open-ended spanner.
■ When loosening a flare nut, or tightening it to a hose, the hexagon end of the hose should be held with a spanner to prevent the tube from twisting and being damaged.
figure 25.8 The parts indicated by the asterisks (*)
should be lubricated with rubber grease during reassembly TOYOTA
figure 25.9 Removing and replacing a hydraulic brake
hose TOYOTA
Replacement brake line
If a brake line is damaged in any way, then a replacement part should be fitted The steel tubing from which brake lines are made is a special double- wall tubing treated against corrosion The ends of the tubing are flared or finished in a particular way to provide a leak-proof joint (Figure 25.10).
■ The special steel tubing must be used for ment brake lines Copper tubing is unsuitable and must not be used.
Trang 7replace-Bleeding brakes
Whenever any part of the hydraulic system has been
disconnected, air will enter the system This will cause
the brakes to be spongy This means that the pedal will
have a soft springy effect because air in the system is
compressed Where a large amount of air has entered
the system, it will be found that the pedal can be
depressed right to the floor without any resistance.
To remove the air that is trapped in the system, the
brakes have to be bled The procedure involves
drawing fluid from the bleeder valve at each wheel
cylinder (or at each caliper) until the air is removed
from the system.
There are two methods that can be used: manual
bleeding and pressure bleeding Manual bleeding uses
the master cylinder to pump the fluid through the
system Pressure bleeding uses a pressure container of
some kind to force fluid through the system.
Manual bleeding
Before commencing to bleed, check the master
cylinder reservoir and top up if necessary The fluid
must be maintained at a reasonable level during
bleeding to prevent air from being drawn into the
master cylinder.
■ It might be necessary to top up the reservoir several
times before bleeding is completed.
The procedure for manual bleeding is as follows:
1 Attach a plastic tube to the end of the bleeder valve
at the caliper or wheel cylinder Place the open end
of the tube in a glass container that is partly filled
with brake fluid The end of the tube should be
submerged in the fluid to prevent air from re-entering the system (Figure 25.11).
2 Unscrew the bleeder valve one-half to quarters of a turn This will allow the fluid to flow from the wheel cylinder.
three-3 Have an assistant apply the footbrake slowly a few times This will pump fluid through the system and out the open bleeder valve, taking any air with it The air will appear as bubbles in the fluid in the container.
4 When bubbles cease to appear and only fluid is being discharged, tighten the valve and remove the tube.
■ The valve should be tightened during a pedal stroke to prevent the possible entry of air.
down-5 Before proceeding with the next wheel, check the fluid level in the reservoir and top up if necessary, but do not overfill Do not reuse fluid that has been bled from the system.
6 When all the brakes have been bled, depress the brake pedal several times and then hold it down If all the air has been removed, the pedal should feel firm and there should be plenty of pedal reserve If the pedal is spongy, indicating that air is still in the system, bleeding will have to be done again.
Other points about bleeding
With some systems that are difficult to bleed, close off the bleeder valve at the end of each pedal downstroke This ensures that air will not be drawn back into the system during the upstroke The bleeder valve is reopened at the start of each downstroke.
figure 25.10 Two types of flared ends that are used on
hydraulic brake lines
figure 25.11 Bleeding the brakes with a tube connected to
the bleeder valve on the caliper
Trang 8With split systems, each circuit is bled separately.
The longest line of the circuit should be bled first, then
the shorter line of that circuit The other circuit is
then bled in the same way.
Where the master cylinder has been dismantled, it
should be primed on the bench It can then be bled
after being installed on the vehicle and before the line
connections are tightened.
Pressure bleeding
Pressure bleeders have a container of fluid and a
means of applying pressure The pressure is applied by
either a hand pump or a workshop air line The pump
is fitted with a plastic hose that is connected to an
adaptor on top of the master cylinder reservoir.
Fluid at low pressure is supplied to the reservoir
and this can be bled from the bleeder valves The pedal
does not have to be pumped.
With pressure bleeders, care must be taken that the
fluid is not exposed to the atmosphere for long periods.
There is some advantage in using small containers of fluid that can be easily closed with screw caps.
Master cylinder service
Figure 25.12 has four different views of the dismantled parts of a master cylinder (The complete cylinder can
be seen in Figures 24.12 and 24.13 in the previous chapter.)
The following paragraphs, which relate to Figure 25.12, provide an appreciation of the main operations that are carried out when working on a master cylinder:
1 Proportioning valve The proportioning valve is
an assembly that is part of the master cylinder (Figure 25.12(a)) It fits into a bore in the cylinder body and can be removed after removing the end plug from the front of the cylinder.
The O-rings on the valve assembly can be renewed, but the proportioning valve assembly
figure 25.12 Dismantled parts of a tandem master cylinder
reservoir seals
outlets
reservoir
circlip fast-fill valve O-ring
plug O-ring
proportioning valve
master cylinder body
(a) Proportioning valve assembly
(c) Primary piston assembly (d) Master cylinder body
(b) Secondary piston assembly
retainer
primary cup
primary piston
O-ring
recuperating guide
master cylinder
secondary spring
secondary piston stop pin secondary piston
caged spring
Trang 9cannot be dismantled If it has a fault, then the
complete valve assembly has to be replaced.
When being reinstalled into its bore, care should
be taken to prevent the O-rings on the valve from
being damaged.
2 Secondary piston The secondary piston assembly is
not dismantled (Figure 25.12(b)) Normally, if the
seals are worn or damaged, a new piston assembly
has to be fitted.
When being installed in the cylinder, the piston assembly is pushed carefully to the bottom of the
cylinder bore with a soft wooden dowel It is fitted
with the caged spring towards the open end of the
cylinder.
The piston is retained in the cylinder by a stop pin which is accessed through the reservoir front
inlet port The pin fits through a slot in the piston,
which must be aligned with the pin holes in the
cylinder during reassembly.
3 Primary piston The parts of the primary piston
assembly are shown (Figure 25.12(c)) The large
O-ring is fitted in a groove in the large end of the
stepped piston The recuperating guide, L-type seal,
and the seal retainer are fitted to the small end of
the piston.
■ When assembling and installing, the lips of the seal
must face into the cylinder.
4 Master cylinder body This shows the seals between
the reservoir and the cylinder (Figure 25.12(d)).
The reservoir is secured to the cylinder by a bolt.
The fast-fill valve is located in the cylinder reservoir rear inlet port area, and is accessible with
the reservoir removed It is retained in place by a
circlip.
General master cylinder service points
When dismantling a master cylinder, care should be
taken to avoid damage to the parts O-rings and seals
should be carefully removed from pistons and valves
so that the grooves into which they fit are not
damaged.
When cleaning a master cylinder, the small hole in
the compensating port should be carefully checked to
ensure that it is not blocked A small probe can be
used to clear the hole If this hole is blocked, pressure
will not be relieved from the brake system when the
brake pedal is released, and binding of the brakes will
occur.
Special rubber grease or clean brake fluid should be
applied to the parts as they are being assembled This will make it easier for the seals and O-rings to enter the bore of the master cylinder.
Drum-brake service
A knowledge of the design features and principles of operation of different drum brakes is of great assistance Brakes are an important safety item, so a high standard of workmanship is required The following points relate to servicing drum brakes.
Access to drum brakes
The brake assembly is accessible after the wheel and the brake drum have been removed In some cases, the brake drum is part of the wheel hub In others, the brake drum is a separate part, designed to fit over the boss of the hub with holes to fit the wheel studs Locating screws which hold the drum to the hub are used with some applications.
The arrangement of the brake drum and hub depends to a large extent on whether the vehicle has front-wheel drive or rear-wheel drive and whether it is
a front-wheel brake or a rear-wheel brake.
Drum-brake construction
The construction of a typical drum brake is shown in Figure 25.13 This is for a rear-wheel-brake assembly with a leading and a trailing shoe Being a rear-wheel- brake assembly, it includes the cable and lever for the parking brake.
The illustration shows the brake assembly in a dismantled condition – the brake drum has been removed from the wheel hub and the brake shoes have been removed from the backing plate The wheel cylinder has also been removed from the backing plate and is shown in a dismantled condition.
Rear-wheel drum brakes are self adjusting by means of a threaded strut between the brake shoes and
an adjusting lever on the rear shoe These parts are also shown dismantled.
■ The illustration should be used to identify the various parts and consider how they fit together when reassembled.
Dismantling drum brakes
Before dismantling and also during dismantling, any features that will assist when reassembling should be noted Some designs, such as duo-servo brakes, have
Trang 10primary and secondary shoes that must be replaced in
their correct positions.
In some instances, the shoes might be different, the
linings might be of different lengths or of different
materials, or the return springs might be of different
lengths or strengths.
With some brake assemblies, there may appear to
be alternative holes in the brake shoes where the return
spring can be attached The correct hole can often be
identified by the ‘witness’ mark worn on the brake
shoe web by the end of the spring.
■ Before dismantling, the front and rear shoes of each
wheel should be identified and, if necessary, marked
to avoid possible confusion when reassembling.
Dismantling sequence
Figure 25.14 shows the general sequence of
dismantl-ing a rear-wheel drum-brake assembly The drum has
already been removed.
1 Releasing the shoes The return spring, which is
located between the shoes, is removed to release the
shoes (Figure 25.14(a)) There are special tools that can assist by getting behind the hook of the spring One type of tool is shown.
2 Removing the front shoe The retaining spring is removed to free the front shoe (Figure 25.14(b)) The spring is held against the shoe by a pin and a retainer A special tool is being used to compress the spring and turn the slotted retainer so that it can
be removed from the pin Pliers can be used instead
of the special tool.
3 Removing the rear shoe The retaining spring is removed to free the rear shoe from the backing plate (Figure 25.14(c)) This shoe has the parking- brake lever and cable still attached.
4 Removing the adjusting-lever spring The adjusting lever has a small return spring This is now removed (Figure 25.14(d)).
5 Removing the strut The adjuster strut can be separated from the rear shoe by unhooking the spring from the shoe (Figure 25.14(e)).
figure 25.13 Components of a rear drum-brake assembly that include the parking brake and the self-adjusting device
adjusting lever spring
C-washer
strut with spring and adjuster
anchor spring cup
retaining spring
retainer
front shoe
handbrake cable hub
backing plate
rear shoe
brake drum pin
plug
Trang 116 Disconnecting the parking-brake cable The cable
is unhooked from the lever using long-nose pliers
(Figure 25.14(f)) The pliers are used to push and
hold the cable return spring away from the lever.
This exposes the end of the cable, which can then
be unhooked from the end of the lever.
The parking brake lever and the adjusting lever fit on to a pin at the top of the brake shoe They can
be removed by expanding the C washers that are
used to retain them in place.
Cleaning
Brake linings should be free of grease and brake fluid.
Brake shoes and linings should not be immersed in
cleaning fluid, but can be cleaned dry.
Linings that have become contaminated with grease, or which are fluid-soaked, must be replaced It
is not possible to clean fluid-soaked linings.
A small amount of grease or fluid on the linings will produce a gummy surface which will cause the brake to grab, while fluid-soaked linings on one brake can reduce friction to the extent that the opposite brake will appear to grab.
Drums and other brake parts collect dust from lining wear, some of which could contain asbestos This dust must be removed carefully, preferably with a vacuum cleaner.
■ Compressed air should never be used to blow brake-lining dust off the parts Breathing dust is a
figure 25.14 Rear drum brake – removing the brake shoes from the backing plate TOYOTA
(a) Removing the return spring
(c) Removing rear shoe
(e) Removing the strut
(b) Removing front shoe
(d) Removing adjusting-lever spring
(f) Disconnecting the parking-brake cable