GMC 4000 Series Service Manual page 204

TYPE "FR -3A " REAR BRAKE
The type "FR-3A" rear brake is the same as
the type "FR-3" brake except for method of ad­
justment. The type "FR-3A" brake has an auto­
matic adjusting mechanism. Refer to figure 43.
Brake shoe adjustment takes place when the
brakes are applied with firm pedal effort while
vehicle is backing up. When brakes are applied
(backing up) the heel of the brake shoe moves away
from the forward-acting anchor, an action which
places a cocking motion in the link-crank system .
An adjustable eccentric on the shoe web provides
a hinge for a short link which carries this motion
to an adjuster crank fastened on the forward acting
anchor pin by a C-washer. From the adjuster
crank, motion transfers through the long link to
the star wheel crank assembly mounted on the op­
posite anchor bracket so that a pawl on the crank
m eshes with the star wheel. The motion pivots
(cocks) the crank back, the force overcoming the
adjuster spring connected between a finger of the
crank and the brake shoe return spring pin. If the
lining clearance perm its sufficient movement, the
crank pawl picks up the next tooth on the star wheel.
Unintentional back-off of the star wheel is
prevented by a friction ring located on the star
wheel screw. This ring applies sufficient drag to
prevent an automatic back-off, but not enough to
prevent manual adjustments at the star wheel.
Automatic adjustment is completed upon brake
release, as the adjuster spring returns the star
wheel crank, advancing the star wheel one tooth.
Completion may be delayed by anchoring pressure
against the star wheel screw; in this case, it is
completed as the anchor pressure is relieved by
the next forward brake application.
BRAKE SHOE REMOVAL
(Refer to Figure 44)
1. Jack up axle and remove hub and brake
drum assembly as directed in "REAR HUBS AND
BEARINGS" (SEC. 4C) of this manual. It m aybe
necessary to back off adjuster star wheels slightly
to free grooved drums.
CAUTION: DO NOT back off adjust­
ment so much that star wheel is jammed
against the friction ring on the star wheel
screw; this may damage the friction ring.
2. Unhook the two automatic adjuster springs.
3. Remove the two long crank links by pivoting
back the star wheel cranks until their slots align
with the link "U" hooks. Lift out links, then slide
their "S" hooks from the adjuster cranks.
G M C S E R V I C E M A N U A L
juster cranks until link "U" hooks clear the eccen­
trics on shoe webs, then remove the sm aller "U"
hooks from the adjuster cranks.
off cranks.
to anchor support and remove crank.
centric from brake shoe.
two short return springs by sliding looped ends off
pins.
washers from hold-down bolts and lift off brake
shoes.
anchor support from shoe side of support. Lift
star wheels from support slots.
ring from a star wheel screw; if necessary, re­
place with a new screw and friction ring assembly.
HYDRAULIC BRAKES
1 Brake Shoe and
Lining Assembly
2 Hold-down Bolt,
Washer, Nut, and
Lock Wire
3 Crank Link (Long)
4 Eccentric and Screw
5 Shoe Return Spring
(Short)
6 Adjuster Crank
7 Crank Link (Short)
Figure 4 3 — Type "FR -3A " Rear Brake Installed
4. Remove short crank links by rotating ad­
5. Spread adjuster crank C-washers and lift
6. Remove bolt which fastens star wheel crank
7. Remove adjuster eccentric screw and ec­
8. Remove two long shoe return springs and
9. Remove lock w ires, hold-down nuts, and
10. Thread each star wheel screw out of
NOTE: DO NOT attempt to remove a friction
Sec. 5A
Page 197
8 Wheel Cylinder
Cover
9 Wheel Cylinder
10 Backing Plate
11 Adjuster Spring
12 Starwheel
13 Starwheel Crank
14 Shoe Return Spring
(Long)
15 Anchor Pin
16 "C" Washer
T-4579