GMC 4000 Series Service Manual page 171

Sec. 5A
Page 164
HYDRAULIC BRAKES
Type "FR-3" (Fig. 3)
1. Jack up vehicle until wheels are clear of
floor. Remove adjusting hole covers from backing
plate.
2. At one adjusting slot, insert adjusting tool
(J-4707) or sim ilar tool through slot and engage
adjusting wheel and decrease lining clearance until
lining drags on drum.
3. Relieve drag by rotating adjusting wheel in
opposite direction. Back off adjustment as follows:
For worn lining - 3 notches (clicks).
For new lining - 5 notches (clicks).
4. At other adjusting slot, repeat Steps 2 and
3 to adjust other shoe.
5. Install hole covers in backing plate.
AUTOMATIC ADJUSTMENT
Duo-Servo
As the brakes are applied, when vehicle is
traveling in reverse, the shoes contact the drum
and revolve with the wheel until the web of the
primary shoe contacts the anchor pin. This motion
causes the secondary shoe to move away from the
anchor pin, thus increasing the distance between
the actuating lever pivot point and the anchor pin.
Since the actuating link maintains a constant length,
the actuating lever assembly pivots on the second­
ary shoe hold-down pin location and the pawl end
of the lever rocks down against the adjusting screw
star wheel. If the lining is worn enough to allow
sufficient movement of the secondary shoe, the
pawl will advance the star wheel one or two teeth.
If the brake linings are not worn enough to require
an adjustment, the restricted movement of the
secondary shoe prohibits over-adjustment.
Figure 4 — Backing Off Adjusting Screw (Cross-Section View)
G M C S E R V I C E M A N U A L
springs return the shoes to the rest position. At
the same time, the actuating lever return spring
moves the linkage to the rest position. The contour
of the actuating lever pawl allows the lever to
ratchet back to the rest position and in effect "take
a new bite" on the star wheel. As a result of this
action, the linkage is in position to make the next
adjustment as it is needed.
necessary to make any manual adjustment to this
type brake. However, when it is necessary to re­
move a brake drum it may also be necessary to
"back-off" the adjustment in order to pull edge of
drum past linings. In such cases, insert a screw ­
driver or awl through hole in backing plate and
hold adjusting lever or pawl away from adjusting
screw. Then turn screw with proper brake tool to
loosen adjustment. See figures 4 and 5.
Twin-Action
are applied with a firm pedal effort while the ve­
hicle is backing up. Applying the brakes moves
adjusting levers which turn the star wheels and
rotate the adjusting screws outward from the
anchor brackets. This action adjusts the shoe until
clearance between the lining and drum is within
proper lim its.
recommended that numerous forward and reverse
stops be performed with a firm pedal effort until a
satisfactory pedal height results.
These holes are for service purposes in the event
retracting of the brake shoes is required to remove
the drum. In order to back off the adjusting screws,
Figure 5 — Backing Off Adjusting Screw (Outside V iew )
When the brakes are released, the pull-back
Under normal operating conditions it is not
Brake shoe adjustment takes place when brakes
Should low pedal heights be encountered, it is
A ccess holes are located in the flange plate.