GMC 4000 Series Service Manual page 240

In the event of an automatic emergency ap­
plication (loss of air pressure) and it is necessary
to move the vehicle before air pressure can be
restored, the emergency parking spring can be
com pressed mechanically to release brake. A
spring "caging" tool is part of the chamber assem ­
bly. Remove nut and release stud from its storage
place on chamber body. Remove spring housing
rubber cap and insert stud in hole. Secure stud in
place (1/4 turn) and "cage" spring by tightening
nut with wrench. Directions are also given for this
operation on the body of the chamber (see fig. 23).
This same procedure is followed if the chamber is
to be removed from the vehicle for service.
A manual emergency application (or parking
application) may be made by the driver by pulling
out the knob on the instrument panel which con­
trols manual application. Release of a parking or
emergency application can be made by pushing in
this same knob, provided there is at least 45
pounds pressure in the air brake system .
BRAKE CHAMBER REPLACEMENT
REMOVAL
1. Block vehicle wheels.
2. With brakes released, remove "spring-
caging" tool from its storage place. Remove spring
housing rubber cap, insert stud in hole and turn
1/ 4 turn. Run nut down on stud until finger tight,
then use wrench to turn nut at least three turns.
Spring is now "caged."
NOTE: If both chambers are to be removed,
perform step 2 on each BEFORE proceeding further.
BRAKE SHOES
(Refer to Figure 24)
One-piece molded lining is riveted to each
shoe. Lining should be replaced before it becomes
worn to the extent that the rivets will damage the
brake drum. Make sure new lining fits firmly
against shoe and that all rivets are properly upset.
When brake drums have been machined oversize,
shims should be used between lining and shoe or
oversize lining used to maintain proper lining-to-
drum contact. Refer to "Brake Drums" in this se c ­
tion. Maximum braking efficiency can be obtained
immediately if linings are trued-up with a conven­
tional lining grinder so they are properly central­
ized in relation to center of hub.
Whenever any part of the brake assembly has
been removed and replaced, adjust brakes as di­
rected under "Brake Adjustments."
G M C S E R V I C E M A N U A L
FRONT BRAKE SHOES
3. Open air tank drain valves and exhaust
pressure from brake system .
4. Disconnect air lines at chamber.
NOTE: If only the emergency parking chamber
needs to be removed, this may be done at this point
by removing rear clamp ring and lifting rear cham­
ber assembly off front (service) chamber.
5. Disconnect push rod clevis from slack ad­
juster.
6. Remove nuts and lock washers from mount­
ing studs.
7. Remove "Anchorlok" brake chamber from
mounting bracket.
INSTALLATION
1. Place "Anchorlok" chamber in position on
mounting bracket.
2. Install lock washers and nuts on studs.
3. Connect push rod clevis to slack adjuster.
4. Connect air lines at chamber.
5. Start engine and build up air pressure to
operating level. Make sure knob is pushed in on
emergency parking brake valve on instrument panel.
6. Back off nut from "caging" stud in end of
chamber. Remove stud and replace in storage
pocket on chamber housing. Install rubber cap
over release stud hole.
7. Check operation of brakes.
8. Check for leaks.
A N D ANCHOR PINS
ANCHOR PINS
Anchor pin ends of shoes are secured on the
anchor pins by lock rings. Shoe ends are not equip­
ped with bushings; if shoes or anchor pins become
worn, parts must be replaced. Contact between
cam ends of shoes and cam is made through ro ll­
ers. No lubricant should be applied at rollers or
cam head; anchor pin ends should be coated with
S-17 Special Lubricant or equivalent at assembly.
Guide pins, two for each shoe, hold shoes in align­
ment on backing plate.
CAMSHAFTS
Front brake camshafts have constant lift type
cams forged integral with shaft. Camshafts are
mounted in camshaft and brake chamber brackets
which are bolted to backing plate (fig. 24). Each
Sec. 5B
Page 233
AIR BRAKES