GMC 1976 ZEO 6083 Maintenance Manual page 227

5- 8 BRAKES
ALL BRAKES DRAG OR PEDAL
BUILDS UP WITH USE AFTER
ADJUSTMENT IS CHECKED AND
FOUND TO BE CORRECT
Cause
Mineral oil, etc., in system .
Pedal does not return freely .
Compensating port of master
cylinder closed .
LOUD LOW PITCH SQUEAL AT
END OF HIGH RATE STOP
Cause
New linings not fully burnished.
Angle on shoe web at adjusting
screw notch.
Bent backing plate. Top of shoe
webs should be in line with each
other looking down on them .
Check after pushing shoes toward
backing plate at top.
Incorrect adjustment .
The presense in the hydraulic system of any miner-
al oil, kerosene, gasoline, shock absorber or
transmission fluid, or carbon tetrachloride will
cause swelling of rubber piston cups, and valves,
so they become inoperative. This is first noticed
in the master cylinder . Brakes will not release
freely if master cylinder primary piston cup has
swollen sufficiently to obstruct the compensating
port . Flush system thoroughly with a good grade of
clean brake fluid and replace all internal rubber
parts in brake system .
Lubricate pedal linkage and make certain no bind
exists . See that stop light switch is not defec-
tive, incorrectly adjusted, or that switch plunger
is not binding on pedal due to lack of lubrication .
The compensating port in master cylinder must be
completely clear when pedal is in released
position .
1 . See that pedal returns freely and is not
stopped by contact with stop light switch body or
pedal bracket.
2 . See that compensating ports are not plugged by
dirt . To check compensating port, remove master
cylinder reservoir cover and watch the fluid in
the cylinder as the brake pedal is moved. A
"geyser" should be seen as the pedal is first de-
pressed. If no geyser is seen, the compensating
port is blocked.
3 . Inspect master cylinder primary piston cup and
if found to be swollen or elongated, flush system
and replace damaged parts.
Check adjustment . Sand lining high spots.
File straight.
Straighten or replace.
Remedy
Remedy
Adjust brakes .
(NOTE: Drum springs not effective against low
pitch squeal or howl .)