GMC 1986 P-Truck Service Manual page 462

5-42 BRAKES
of these tolerances provide the surface necessary to
prevent brake roughness.
Light scoring of the rotor surface not in excess of
0.38 mm (0.15-inch) in depth is normal. This condition
does not affect the brake operation.
LATERAL RUNOUT
Lateral runout is the movement of the rotor from side
to side as it rotates on the spindle. This could also be
refered to as " rotor wobble" .
This movement causes the brake pad and piston to
be knocked back into it's bore. This results in
additional pedal travel and a vibration during braking.
Checking Lateral Runout (Figure 47)
1. Tighten the wheel bearings to eliminate all
freeplay.
2. Attach a dial indicator to some portion of the
suspension.
• The point of the styles must contact the rotor face
about 25 mm (1-inch) from the rotor edge.
3. Move the rotor one complete rotation.
• The lateral runout should not exceed 0.10 mm
(0.004-inch).
4. Readjust the wheel bearings, refer to FRONT
SUSPENSION (Sec. 3C).
PARALLELISM
Parallelism is the measurement of the thickness of
the ro to r at fo u r or m ore p o in ts a round the
circumference of the rotor. All measurements must be
made at the same distance in from the edge of the
rotor. The rotor thickness must not vary more than
0.013 mm (0.0005-inch) from point to point.
MACHINING
Since accurate control of the rotor tolerances is
necessary for proper performance of the disc brakes,
machining of the rotor should be done only with
precision equipment.
A ll brake rotors have a m inim um th ic k n e s s
dimension cast into them. This dimmension is the
m in im um w ear d im e n s io n and not a re fin is h
dimension. Do not use a brake rotor that will not meet
the specifications, after refinishing. Replace with a new
brake rotor. Refer to " Specification" in this section for
final maching tolerances.
REBUILDING THE CALIPER
3000/3100 MODELS
* Disassemble (Figure 48)
• Drain all the fluid from the caliper.
• Pad the interior of the caliper with clean shop
towels.
CAUTION: Do not place your fingers in fro n t of
the piston in an attem pt to catch or protect it
when applying compressed air. This could
result in serious injury.
NOTICE: Use just enough air to ease the
piston out of the bore. If piston is blown
out—even with padding provided, it may be
damaged.
1. Piston (22) (figure 49)
2. Boot (23) (figure 50).
• Use care not to scratch the housing bore.
3. Piston seal (21).
• Do not use any type of metal tool.
4. Bleeder valve (20).
T jjf
Clean
• Bleeder valve, caliper bore, caliper passages, and
piston with denatured alcohol. Use dry, filtered
compressed air to dry parts and blow out
passages.