Chevrolet 1977 light duty truck, Service Manual

Page: 428 / 861

pro b ab ly be necessary to rebore in order to true up the
brak in g surface.
If the brake linings are slightly worn and the drum
is grooved, the d ru m should be polished with fine em ery
cloth but should not be turned. At this stage, elim inating
the grooves in d rum would necessitate removal o f too
much metal, while if left alone, the grooves and lining
ridges match and satisfactory service can be obtained.
If brake linings are to be replaced, a grooved drum
should be turned for use w ith new linings. A grooved
drum , if used with new lining, will not only w ear the
lining, but will make it difficult, if not impossible to
o b ta in efficient brake p e rform ance.
Out-Of-Round or Tapered Drum
An out-of-round drum makes accurate brake shoe
adjustm ent im possible and is likely to cause excessive
wear o f other parts o f brake m echanism due to its
eccentric action. An out-of-round drum can also cause
severe and irregular tire tread w ear as well as pulsating
brake pedal. W h e n the b rak in g surface o f a brake d rum
exceeds the factory specification limits in ta p e r ( a n d /o r )
being out-of-round, the d ru m should be turned to true
up the b rak in g surface. O ut-of-round as well as taper
and w ear can be accurately measured with an inside
m icrom eter fitted with p ro p er extension rods.
W h e n m easuring a d ru m for out-of-round, taper
and wear, take m easurem ents at the open and closed
edges o f m ach in ed surface and at right angles to each
other.
Turnin g Brake Drums
If a d ru m is to be turned, only enough metal should
be rem oved to o btain a true, smooth braking surface. If
a d ru m does not clean-up when turned to a maxim um
d ia m e te r as shown in the general specification, it must be
replaced. Removal o f more metal will affect dissipation
of heat and m ay cause distortion o f the drum .
All brake dru m s have a m axim um d iam eter cast
into them. This d ia m e te r is the m axim um w ear d iam eter
and not a refinish diam eter. Do not refinish a brake
d ru m that will not meet the specifications, as shown
below, after refinishing.
O R IG IN A L
DRUM DIAMETERS
M A X IM U M R E F IN IS H
R E P L A C E M E N T
(D IS C A R D )
11.000
11 .0 6 0 11 .0 9 0
12.000 12.0 60 12.0 9 0
13.000 13.0 60 13 .0 9 0
Brake Drum Balance
D uring m anufacture, brake drum s are balanced
within three ounce inches. These weights must not be
removed.
WHEEL CYLINDER
Removal
1. Remove wheel, d rum and brake shoes. Be
careful not to get grease or dirt on brake lining.
2. Remove wheel cylinder from backing plate.
Disassembly
1. Inspect cylinder bore for scoring or corrosion. It
is best to replace a corroded cylinder.
2. Polish any discolored or stained are a with crocus
cloth by revolving cylinder on cloth supported by a
finger.
3. Rinse cylinder in clean brake fluid.
4. Shake excessive rinsing fluid from cylinder. Do
not use a rag to dry cylinder, as lint from the rag cannot
be kept from cylinder bore surfaces.
Assembly
1. Lubricate cylinder bore and counterbore with
clean brake fluid and insert spring-expander assembly.
2. Install new cups. (Be sure cups are lint and dirt
free.) Do not lubricate cups prior to assembly.
3. Install new pistons.
4. Press new boots into cylinder counterbores by
hand. Do not lubricate boots prior to assembly.
Installation
1. Install wheel cylinder on brake backing plate
and connect brake pipe to hose. Torque rear wheel brake
pipe to wheel cylinder to specifications.
2. Install brake shoes, drum and wheel; then flush
and bleed hydraulic system.
POWER BRAKE UNIT
Vacuum Booster
Replacement (Fig. 5 -5 4 )
C A U TIO N : See "C aution" on page 1 o f this
section when installing fasteners referred to in
steps 6, 7 and 9 below.
1. Remove two nuts holding m aster cylinder to
power cylinder and position it away from power
cylinder.
C A U T IO N :
Do not disconnect hydraulic brake
lines; be careful not to bend or k in k pipes.
2. Disconnect the vacuum hose from the vacuum
check valve on the fro n t housing o f the power head. Plug
vacuum hose to prevent dust and dirt fro m entering
hose.
3. Disconnect the power brake push rod from the
brake pedal.
4. Remove the four nuts from the m ounting studs
which hold the power brake to the cowl.