Chevrolet 1980 Light Duty Truck Service Manual page 388

5-34 BRAKES
Fig. 5-43--C hecking Rotor for Lateral Runout
4. Lateral runout should not be over .004" (0.10 mm)
total indicator reading.
Parallelism C heck
1. Parallelism is the measurement of the thickness of
the rotor at four or more points around the circumference
of the rotor. All measurements must be made at the same
distance in from the edge of the rotor.
2. A rotor that varies over .0005" (0.013 mm) causes
pedal vibration, as well as front end vibration during brake
applications. A rotor that does not meet these specifications
may be refmished to specifications if precision equipment
is available.
Rotor T o le ra n c e and S u rfa c e Finish
In manufacturing the brake rotor, tolerances of the
rubbing surfaces for flatness, for parallelism and for lateral
runout are held very closely. The maintenance of these close
controls on the shape of the rubbing surfaces is necessary
to prevent brake roughness.
In addition to these tolerences, the surface finish must
be held to a specified range. The control of the rubbing
surface finish is necessary to avoid pulls and erratic
performance and to extend lining life.
Light scoring of the rotor surfaces not exceeding .015"
(0.38 mm) in depth, which may result from normal use, is
not detrimental to brake operation.
M a ch in in g
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.
C A U TIO N : All brake
minimum thickness dimension cast into
them. Th is dimension is the minimum
w ear dimension
dimension. Do not use a brake rotor that
will not meet the specifications, as show n
below, after refinishing. Replace w ith
new brake rotor.
rotors have a
and not a refinish
ROTOR THICKNESS
MINIMUM
AFTER REFINISHING
1.480
1.230
.980
W h e e l Bolt R ep lac e m en t (Disc B rakes)
See "Notice" on page 1 of this section.
N O TICE:
1. Remove hub and rotor assembly from vehicle.
2. Mark rotor to hub location and remove bolts
attaching hub to rotor.
3. The wheel bolts on disc brakes can be pressed out
from the outside of the hub and installed from inside the
rotor by pressing into place. No drilling or cutting is
required.
4. Reinstall assembly and adjust wheel bearings.
DRUM B R A K E SH O E AND LINING
Service Brake
A d ju s tm e n t
1. Using a punch, knock out lanced area in brake
drum. If this is done with the drum installed on the vehicle,
the drum must be removed and all metal cleaned out of the
brake compartment. Be sure to procure a new metal hole
cover and install it in the drum after adjustment to prevent
dirt and water from getting into the brakes. Use J-6166 to
turn brake adjusting screw; expand brake shoes at each
wheel until the wheel can just be turned by hand. The drag
should be equal at all wheels.
2. Back off brake adjusting screw at each wheel 30
notches. If shoes still drag lightly on drum, back off
adjusting screw one or two additional notches. Brakes
should be free of drag when screw has been backed off
approximately 12 notches. Heavy drag at this point
indicates tight parking brake cables.
3. Install metal adjusting hole cover in brake drum.
4. Check parking brake adjustment.
R em oval and In specting
See "Notice" on Page 1 of this section.
N O TICE:
1. Raise vehicle on hoist. Remove wheel and brake
drum. It may be necessary to back off the brake shoe
adjustment before the brake drums can be removed. To back
off shoe adjustment, rotate shoe adjusting screw downward.
On JB6, JB7 and JB8 equipped vehicles, it is necessary
to remove the axle shafts to remove the hub and drum
assembly; refer to Section 4B of this manual.
2. Unhook the primary and secondary shoe return
springs using large pliers.
3. Remove shoe hold down springs.
4. Lift up on actuator, unhook actuating link from
anchor pin, then remove.
REPLACEMENT
(DISCARD)
1.465
1.215
.965
LIG H T T R U C K S ER V IC E M AN UAL