Chapter
9 Brakes
9-
1 General information
Refer to illustration 7.2
The vehicles covered by this manual are equipped with hydraulically
operated front and rear brake systems. The front brakes are disc type
and the rear brakes are drum type. Both the front and rear brakes are
self adjusting. The front disc brakes automatically compensate for pad
wear, while the rear drum brakes incorporate an adjustment mechanism
which is activated as the brakes are applied when the vehicle is driven
reverse.
Hydraulic system
The hydraulic system consists of t w o separate circuits (see illustra-
tion). The master cylinder has separate reservoirs for the t w o circuits
and in the event of a leak or failure in one hydraulic circuit, the other
circuit will remain operative.
A
visual warning of circuit failure or air
in the system is given by a warning light activated by displacement
of the piston in the pressure differential switch portion of the com-
bination valve from its normal "in balance" position.
Combination valve
A combination valve, located in the engine compartment below the
master cylinder, consists of three sections providing the following func-
tions. The metering section limits pressure t o the front brakes until
a predetermined front input pressure is reached and until the rear brakes
are activated. There is no restriction at inlet pressures below
3
psi,
allowing pressure equalization during non-braking periods. The propor-
tioning section proportions outlet pressure t o the rear brakes after a
predetermined rear input pressure has been reached, preventing early
rear wheel lock-up under heavy brake loads. The valve is also designed
t o assure full pressure to one brake system should the other system
fail. The pressure differential warning switch incorporated into the com-
bination valve is designed t o continuously compare the front and rear
brake pressure from the master cylinder and energize the dash warning
light in the event of either a front or rear brake system failure. The
design of the switch and valve are such that the switch will stay in
the Warning position once a failure has occurred. The only way t o turn
the light off is t o repair the cause of the failure and apply a brake pedal
force of
450
psi.
Power brake booster (non-ABS models)
The power brake booster, utilizing engine manifold vacuum and at-
mospheric pressure t o provide assistance to the hydraulically operated
brakes, is mounted on the
in the engine compartment.
Parking brake
The parking brake operates the rear brakes only, through cable ac-
tuation. It's activated by a lever mounted in the center console
(Cherokee models) or a pedal mounted on the left side kick panel (Com-
anche models).
Service
After completing any operation involving disassembly of any part
of the brake system, always test drive the vehicle t o check for proper
braking performance before resuming normal driving. When testing the
brakes, perform the tests on a clean, dry flat surface. Conditions other
than these can lead t o inaccurate test results.
Test the brakes at various speeds with both light and heavy pedal
pressure. The vehicle should stop evenly without pulling t o one side
or the other. Avoid locking the brakes because this slides the tires and
diminishes braking efficiency and control of the vehicle.
Tires, vehicle load and front-end alignment are factors which also
affect braking performance.
2
Anti-lock brake system
-
general information
Refer to illustrations
2.2, 2.4
and
2.5
The anti-lock brake system was introduced in
1989
and is designed
t o maintain vehicle
directional stability and optimum
deceleration under severe braking conditions and on most road sur-
faces. It does so by monitoring the rotational speed of each wheel and
controlling the brake line pressure t o each wheel during braking. This
prevents the wheel from locking-up and provides maximum vehicle
controllability.
Components
Actuation
assembly
The actuation assembly consists of the master cylinderipower
Schematic of the brake hydraulic system
1
Master cylinder
3 Disc brake caliper
2
Combination valve
4
Wheel cylinder
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Summary of Contents for cherokee 1989
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