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Jeep Grand Cherokee WJ. Manual - part 49

FRONT DISC BRAKES

DESCRIPTION

The calipers are twin piston type. The calipers are

free to slide laterally on the anchor, this allows con-
tinuous compensation for lining wear.

OPERATION

When the brakes are applied fluid pressure is

exerted against the caliper pistons. The fluid pres-
sure is exerted equally and in all directions. This

means pressure exerted against the caliper pistons
and within the caliper bores will be equal (Fig. 2).

Fluid pressure applied to the pistons is transmit-

ted directly to the inboard brake shoe. This forces the
shoe lining against the inner surface of the disc
brake rotor. At the same time, fluid pressure within
the piston bores forces the caliper to slide inward on
the slide pins. This action brings the outboard brake
shoe lining into contact with the outer surface of the
disc brake rotor.

Fig. 1 Power Brake Booster–Typical

1 – VACUUM CHECK VALVE
2 – FRONT DIAPHRAGM
3 – REAR DIAPHRAGM
4 – HOUSING
5 – SEAL
6 – AIR FILTER

7 – PRIMARY PUSH ROD (TO BRAKE PEDAL)
8 – ATMOSPHERIC INLET VALVE ASSEMBLY
9 – BOOSTER MOUNTING STUDS (4)
10 – SECONDARY PUSH ROD (TO MASTER CYLINDER)
11 – MASTER CYLINDER MOUNTING STUD (2)
12 – SPRING

5 - 4

BRAKES

DESCRIPTION AND OPERATION (Continued)

Fluid pressure acting simultaneously on the pis-

tons and caliper to produces a strong clamping
action. When sufficient force is applied, friction will
stop the rotors from turning and bring the vehicle to
a stop.

Application and release of the brake pedal gener-

ates only a very slight movement of the caliper and
pistons. Upon release of the pedal, the caliper and
pistons return to a rest position. The brake shoes do
not retract an appreciable distance from the rotor. In
fact, clearance is usually at, or close to zero. The rea-
sons for this are to keep road debris from getting
between the rotor and lining and in wiping the rotor
surface clear each revolution.

The caliper piston seals control the amount of pis-

ton extension needed to compensate for normal lining
wear.

During brake application, the seals are deflected

outward by fluid pressure and piston movement (Fig.
3).

When

the

brakes

(and

fluid

pressure)

are

released, the seals relax and retract the pistons.

The front outboard brake shoes have wear indica-

tors.

REAR DISC BRAKES

DESCRIPTION

The rear disc brakes consist of single piston float-

ing-type calipers and solid rotors. The rear caliper is
mounted on an anchor attached to an adapter
attached the rear axle tube flange. The anchors are
secured to the adapters with mounting bolts. The
disc brake rotor splash shield is part of the adaptor.
The disc brake rotor has a built in brake drum used
for the parking brakes (Fig. 4). The parking brake
shoes are mounted to the adaptor.

OPERATION

When the brakes are applied fluid pressure is

exerted against the caliper pistons. The fluid pres-
sure is exerted equally and in all directions. This
means pressure exerted against the caliper pistons
and within the caliper bores will be equal (Fig. 2).

Fluid pressure applied to the pistons is transmit-

ted directly to the inboard brake shoe. This forces the
shoe lining against the inner surface of the disc

Fig. 2 Brake Caliper Operation

1 – CALIPER
2 – PISTON
3 – PISTON BORE
4 – SEAL
5 – INBOARD SHOE
6 – OUTBOARD SHOE

Fig. 3 Lining Wear Compensation By Piston Seal

1 – PISTON
2 – CYLINDER BORE
3 – PISTON SEAL BRAKE PRESSURE OFF
4 – CALIPER HOUSING
5 – DUST BOOT
6 – PISTON SEAL BRAKE PRESSURE ON

Fig. 4 Rear Disc Brake Rotor

1 – PARKING BRAKE DRUM SURFACE
2 – REAR DISC BRAKE ROTOR

BRAKES

5 - 5

DESCRIPTION AND OPERATION (Continued)

brake rotor. At the same time, fluid pressure within
the piston bores forces the caliper to slide inward on
the slide pins. This action brings the outboard brake
shoe lining into contact with the outer surface of the
disc brake rotor.

Fluid pressure acting simultaneously on the pis-

tons and caliper to produces a strong clamping
action. When sufficient force is applied, friction will
stop the rotors from turning and bring the vehicle to
a stop.

Application and release of the brake pedal gener-

The caliper piston seals control the amount of pis-

ton extension needed to compensate for normal lining
wear.

During brake application, the seals are deflected

outward by fluid pressure and piston movement (Fig.
3).

When

the

brakes

(and

fluid

pressure)

are

released, the seals relax and retract the pistons.

The front outboard brake shoes have wear indica-

tors.

PARKING BRAKES

The parking brakes operated by a automatic ten-

sioner mechanism built into the hand lever and cable
system. The front cable is connected to the hand
lever and the equalizer. The rear cables attached to
the equalizer and the parking brake shoe actuator.

A set of drum type brake shoes are used for park-

ing brakes. The shoes are mounted to the rear disc
brake adaptor. The parking brake drum is integrated
into the rear disc brake rotor.

Parking brake cable adjustment is controlled by an

automatic tensioner mechanism. The only adjust-
ment if necessary is to the park brake shoes if the
linings are worn.

BRAKE HOSES AND LINES

DESCRIPTION

Flexible rubber hose is used at both front brakes,

rear brakes and at the rear axle junction block. Dou-
ble walled steel tubing is used. Double inverted style
and ISO style flares are used on the brake lines.

DIAGNOSIS AND TESTING

BASE BRAKE SYSTEM

Base brake components consist of the brake shoes,

calipers, rear park brake drums/rotors, front brake
rotors, brake lines, master cylinder, booster, HCU
and parking brake shoes.

Brake diagnosis involves determining if the prob-

lem is related to a mechanical, hydraulic, electrical
or vacuum operated component.

The first diagnosis step is the preliminary check.

PRELIMINARY BRAKE CHECK

(1) Check condition of tires and wheels. Damaged

wheels and worn, damaged, or underinflated tires
can cause pull, shudder, vibration, and a condition
similar to grab.

(2) If complaint was based on noise when braking,

check suspension components. Jounce front and rear
of vehicle and listen for noise that might be caused
by loose, worn or damaged suspension or steering
components.

(3) Inspect brake fluid level and condition. Note

that the brake reservoir fluid level will decrease in
proportion to normal lining wear. Also note that
brake fluid tends to darken over time. This is
normal and should not be mistaken for contam-
ination.

(a) If fluid level is abnormally low, look for evi-

dence of leaks at calipers, brake lines, master cyl-
inder, and HCU.

(b) If fluid appears contaminated, drain out a

sample to examine. System will have to be flushed
if fluid is separated into layers, or contains a sub-
stance other than brake fluid. The system seals,
cups, hoses, master cylinder, and HCU will also
have to be replaced after flushing. Use clean brake
fluid to flush the system.
(4) Check parking brake operation. Verify free

movement and full release of cables and lever. Also
note if vehicle was being operated with parking
brake partially applied.

(5) Check brake pedal operation. Verify that pedal

does not bind and has adequate free play. If pedal
lacks free play, check pedal and power booster for
being loose or for bind condition. Do not road test
until condition is corrected.

(6) Check booster vacuum check valve and hose.
(7) If components checked appear OK, road test

the vehicle.

ROAD TESTING

(1) If complaint involved low brake pedal, pump

pedal and note if it comes back up to normal height.

5 - 6

BRAKES

DESCRIPTION AND OPERATION (Continued)

(2) Check brake pedal response with transmission

in neutral and engine running. Pedal should remain
firm under constant foot pressure.

(3) During road test, make normal and firm brake

stops in 25-40 mph range. Note faulty brake opera-
tion such as low pedal, hard pedal, fade, pedal pulsa-
tion, pull, grab, drag, noise, etc.

(4) Attempt to stop the vehicle with the parking

brake only (do not exceed 25 mph) and note grab,
drag, noise, etc.

PEDAL FALLS AWAY

A brake pedal that falls away under steady foot

pressure is generally the result of a system leak. The
leak point could be at a brake line, fitting, hose, or
caliper. If leakage is severe, fluid will be evident at
or around the leaking component.

Internal leakage (seal by-pass) in the master cylin-

der caused by worn or damaged piston cups, may
also be the problem cause.

An internal leak in the ABS system may also be

the problem with no visual fluid leak.

LOW PEDAL

If a low pedal is experienced, pump the pedal sev-

eral times. If the pedal comes back up, the most
likely causes are worn linings, rotors, or calipers are
not sliding on the slide pins. The proper course of
action is to inspect and replace all worn component.

SPONGY PEDAL

A spongy pedal is most often caused by air in the

system. However substandard brake hoses can cause
a spongy pedal. The proper course of action is to
bleed the system, and replace substandard quality
brake hoses if suspected.

HARD PEDAL OR HIGH PEDAL EFFORT

A hard pedal or high pedal effort may be due to

lining that is water soaked, contaminated, glazed, or
badly worn. The power booster, check valve, check
valve seal/grommet or vacuum leak could also cause
a hard pedal or high pedal effort.

PEDAL PULSATION

Pedal pulsation is caused by components that are

loose, or beyond tolerance limits.

The primary cause of pulsation are disc brake

rotors with excessive lateral runout or thickness vari-
ation. Other causes are loose wheel bearings or cali-
pers and worn, damaged tires.

NOTE: Some pedal pulsation may be felt during
ABS activation.

BRAKE DRAG

Brake drag occurs when the lining is in constant

contact with the rotor or drum. Drag can occur at one
wheel, all wheels, fronts only, or rears only.

Drag is a product of incomplete brake release.

Drag can be minor or severe enough to overheat the
linings, rotors and park brake drums.

Minor drag will usually cause slight surface char-

ring of the lining. It can also generate hard spots in
rotors and park brake drums from the overheat-cool
down process. In most cases, the rotors, wheels and
tires are quite warm to the touch after the vehicle is
stopped.

Severe drag can char the brake lining all the way

through. It can also distort and score rotors to the
point of replacement. The wheels, tires and brake
components will be extremely hot. In severe cases,
the lining may generate smoke as it chars from over-
heating.

Common causes of brake drag are:
• Parking brake partially applied.

• Loose/worn wheel bearing.

• Seized caliper.

• Caliper binding.

• Loose caliper mounting.

• Mis-assembled components.

• Damaged brake lines.
If brake drag occurs at the front, rear or all

wheels, the problem may be related to a blocked mas-
ter cylinder return port, faulty power booster (binds-
does not release) or the ABS system.

BRAKE FADE

Brake fade is usually a product of overheating

caused by brake drag. However, brake overheating
and resulting fade can also be caused by riding the
brake pedal, making repeated high deceleration stops
in a short time span, or constant braking on steep
mountain roads. Refer to the Brake Drag information
in this section for causes.

BRAKE PULL

Front brake pull condition could result from:
• Contaminated lining in one caliper

• Seized caliper piston

• Binding caliper

• Loose caliper

• Rusty caliper slide surfaces

• Improper brake shoes

• Damaged rotor

• Wheel alignment.

• Tire pressure.
A worn, damaged wheel bearing or suspension

component are further causes of pull. A damaged
front tire (bruised, ply separation) can also cause
pull.

BRAKES

5 - 7

DIAGNOSIS AND TESTING (Continued)

Content .. 47 48 49 50 ..