GMC 4000 Series Service Manual page 245

Sec. 5B
Page 238
AIR BRAKES
adjuster assembly installed. When the plunger a s­
sembly is moved outward to apply the shoe against
the drum, the plunger guide will slide across the
sloping sides of the teeth on the actuator. If the
plunger assembly moves outward and exceeds the
pitch distance, the teeth on the guide will engage
the next teeth on the actuator. When the plunger
returns, the actuator must rotate in order to allow
the assembly to return to the "in" position. This
rotation is caused by the angle of the teeth. As the
actuator rotates, it screw s the adjusting bolt. The
distance for the plunger to move before adjusting
is controlled either by the angle of the teeth, and/
or the number of teeth. This travel establishes the
lining-drum clearance.
The plunger guide has two flat sides, which
are a slip fit in a slot in the adjusting plunger.
This performs two functions: (1) it prevents the
guide from turning, and (2) it prevents the adjust­
ing plunger from turning in the housing.
In order to function properly the adjusting bolt
must not turn. If it rotates with the actuator, no
adjustment would occur.
The combination of the number of teeth on the
outside of the actuator and the number of threads
per inch on the adjusting bolt establishes the actual
Figure 3 0 — Installing A djusting Bolt
G M C S E R V I C E M A N U A L
linear advance or rate of adjustment.
ation of automatic adjuster components are given
in the following paragraphs.
moved for service they should be lubricated as
directed in the following procedures.
whenever brake shoes are relined and inspected
for the general condition of all moving parts. It
is advisable to use new plunger seals and plunger
guide gaskets when removing and installing these
parts.
REMOVAL (Refer to Fig. 28)
as detailed under applicable procedures.
housing.
of spider housing.
INSTALLATION (Refer to Fig. 28)
is installed at proper end of spider so that brake
shoe ends are mounted in proper position. Also
make certain that all parts are properly lubricated.
All threads on all components should be coated
with lubricant to assure smooth, free operation.
The tip of the plunger guide and the entire plunger
should also be coated. See LUBRICATION (SEC. 0)
for type of lubricant to be used.
actuator.
spider housing.
ing. See figure 36.
full length of threads, then back off 1/4 turn so
that screw will not jam and fail to adjust when
assembly is complete. See figure 30.
end of the guide with teeth is installed first and the
flat sides of the guide must fit into the slot in the
adjusting plunger to mesh with the outer teeth of
the actuator.
end of the guide must be toward the plunger seal
end of the housing, otherwise the automatic adjust­
ment feature will not functiom. When properly as­
sembled, if adjusting bolt is turned in one direc­
tion, the brake will adjust and if turned in opposite
direction a ratcheting effect will result. When a s­
sembled wrong, a ratcheting effect will result
Procedures covering the removal and install­
Any time the adjuster assem blies are re­
The adjuster system should be disassembled
1. Remove brake shoes and brake chambers
2. Remove bolt and plunger guide gasket from
3. Remove spring and plunger guide.
4. Unscrew adjusting bolt from actuator.
5. Using a screwdriver, pry plunger seal out
6. Remove actuator and adjusting plunger.
NOTE: Make sure that adjusting mechanism
1. Install new seal on adjusting plunger and
2. Install adjusting plunger and actuator in
3. Tap plunger seal into place in spider hous­
4. Screw adjusting bolt into actuator to the
5. Install plunger guide in spider housing. The
CAUTION: The chamfered notch on the back