Jeep 1996 Grand Cherokee Service Manual - page 158
(5) Vacuum should hold steady. If gauge on pump
indicates vacuum loss, check valve is faulty and
should be replaced.
COMBINATION VALVE
Metering Valve
Metering valve operation can be checked visually
and with the aid of a helper. Observe the metering
valve stem while a helper applies and releases the
brakes. If the valve is operating correctly, the stem
will extend slightly when the brakes are applied and
retract when the brakes are released. If the valve is
faulty, replace the entire combination valve as an
assembly.
Pressure Differential Switch
(1) Have helper sit in drivers seat to apply brake
pedal and observe red brake warning light.
(2) Raise vehicle on hoist.
(3) Connect bleed hose to a rear wheel cylinder
and immerse hose end in container partially filled
with brake fluid.
(4) Have helper press and hold brake pedal to floor
and observe warning light.
(a) If warning light illuminates, switch is operat-
ing correctly.
(b) If light fails to illuminate, check circuit fuse,
bulb, and wiring. The parking brake switch can be
used to aid in identifying whether or not the brake
light bulb and fuse is functional. Repair or replace
parts as necessary and test differential pressure
switch operation again.
(5) If warning light still does not illuminate,
switch is faulty. Replace combination valve assembly,
bleed brake system and verify proper switch and
valve operation.
FRONT DISC BRAKE ROTOR
ROTOR MINIMUM THICKNESS
Rotor minimum usable thickness is 22.7 mm (0.89
in.). Do not resurface a rotor if machining would
cause thickness to fall below this limit.
Measure rotor thickness at the center of the brake-
shoe contact surface. Replace the rotor if worn below
minimum thickness, or if refinishing would reduce
thickness below the allowable minimum.
FRONT ROTOR THICKNESS VARIATION
Variations in rotor thickness will cause pedal pul-
sation, noise and shudder.
Measure rotor thickness at four to six points
around the rotor face. Position the micrometer
approximately 3/4 inch from the rotor outer circum-
ference for each measurement (Fig. 10).
Thickness should not vary by more than 0.013 mm
(0.0005 in.) from point to point on the rotor. Refinish
or replace the rotor if necessary.
FRONT ROTOR LATERAL RUNOUT
Check rotor lateral runout whenever pedal pulsa-
tion, or rapid, uneven brakelining wear has occurred.
The rotor must be securely clamped to the hub to
ensure an accurate runout measurement. Secure the
rotor with the wheel nuts and 4 or 5 large diameter
flat washers on each stud.
Use a dial indicator to check lateral runout (Fig.
11).
Maximum allowable rotor lateral runout is 0.13
mm (0.005 in.).
Fig. 8 Typical Booster Vacuum Test Connections
Fig. 9 Vacuum Check Valve And Seal
ZJ
BRAKES
5 - 9
DIAGNOSIS AND TESTING (Continued)