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5 1 -10
BRAKES
DIAGNOSIS AND TESTING (Continued)
(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 .
DISC BRAKE ROTOR
The rotor braking surfaces should not be refinished
unless necessary.
Light surface rust and scale can be removed with a
lathe equipped with dual sanding discs. The rotor
surfaces can be restored by machining in a disc brake
lathe if surface scoring and wear are light.
Replace the rotor under the following conditions :
" Severely Scored
" Tapered
" Hard Spots
" Cracked
" Below Minimum Thickness
ROTOR MINIMUM THICKNESS
Measure rotor thickness at the center of the brake
shoe contact surface. Replace the rotor if worn below
minimum thickness, or if machining would reduce
thickness below the allowable minimum .
Rotor minimum thickness is usually specified on
the rotor hub. The specification is either stamped or
cast into the hub surface.
ROTOR RUNOUT
Check rotor lateral runout with dial indicator
C-3339 (Fig. 10). Excessive lateral runout will cause
brake pedal pulsation and rapid, uneven wear of the
brake shoes . Position the dial indicator plunger
approximately 25 .4 mm (1 in.) inward from the rotor
edge .
NOTE : Be sure wheel bearing
has
zero end play
before checking rotor runout .
Maximum allowable rotor runout is 0.102 mm
(0.004 in.).
DIAL
INDICATOR
J8905-68
Fig . 10 Checking Rotor Runout And Thickness
Variation
ROTOR THICKNESS VARIATION
Variations in rotor thickness will cause pedal pul-
sation, noise and shudder.
Measure rotor thickness at 6-to-12 points around
the rotor face (Fig. 11).
Position the micrometer approximately 25.4 mm ( 1
in.) from the rotor outer circumference for each mea-
surement .
Thickness should not vary by more than 0.013 mm
(0.0005 in.) from point-to-point on the rotor . Machine
or replace the rotor if necessary.
TAKE MINIMUM
OF 6 MEASUREMENTS
AROUND ROTOR
Fig. 11 Measuring Rotor Thickness
TJ
BRAKE DRUM
The maximum allowable diameter of the drum
braking surface is stamped or cast into the drum
outer edge. Generally, a drum can be machined to a
maximum of 1.52 mm (0.060 in.) oversize. Always
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