GMC 4000 Series Service Manual page 195

Sec. 5A
Page 188
HYDRAULIC BRAKES
1 Wheel Cylinder
2 Brake Shoe Return
Spring
3 Backing Plate
4 Brake Shoe
5 Brake Lining
Figure 2 6 — Type "F" Front Brake Assem bly
BRAKE SHOE REMOVAL (Fig. 26)
1. Jack up axle and remove hub and brake
drum assembly as directed in "FRONT HUBS AND
BEARINGS" (SEC. 3D) of this manual.
2. Remove both brake shoe return springs,
using brake spring pliers.
3. Remove C-washers and flat washer from
each adjusting cam and hold-down stud. Lift shoes
off backing plate.
CLEANING AND INSPECTION
1. Clean all dirt out of brake drum. Inspect
drum for roughness, scoring, or out-of-round.
Replace or recondition brake drum as necessary.
Refer to "Brake Drums" later in this section.
2. Inspect wheel bearings and oil seals as di­
rected in "FRONT HUBS AND BEARINGS" (SEC.
3D) of this manual.
G M C S E R V I C E M A N U A L
6 Brake Shoe Adjusting
Cam
7 Brake Shoe Guide
Washer
8 Brake Shoe Guide
C-W asher
9 Adjusting Cam and
Shoe Guide Stud
10 Shoe Guide Anti-
Rattle Washer
11 Adjusting Cam Spring
3. Check backing plate attaching bolts to make
sure they are tight. Clean all dirt off backing plate.
4. Inspect brake shoe return springs. If broken,
cracked, or weakened, replace with new springs.
5. Check cam and shoe guide stud and friction
spring on backing plate for corrosion or binding.
Cam stud should turn easily with an 8-inch wrench,
but should not be loose. If frozen, lubricate with
kerosene or penetrating oil and work free.
6. Examine brake shoe linings for wear. Lin­
ings should be replaced if worn down close to rivet
heads. Refer to "Brake Shoe Relining" in this
section.
BRAKE SHOE INSTALLATION
1. Install anti-rattle spring washer on each
cam and shoe guide stud, pronged side facing ad­
justing cam.
2. Place shoe assembly on backing plate with
cam and shoe guide stud inserted through hole in
shoe web; locate shoe toe in wheel cylinder piston
shoe guide and position shoe heel in slot in anchor
block.
3. Install flat washer and C-washer on cam
and shoe guide stud. Crimp ends of C-washer to­
gether.
4. After installing both shoes, install brake
shoe return spring. To install each spring, place
spring end with short hook in toe of shoe, then
using brake spring pliers, stretch spring and s e ­
cure long hook end in heel of opposite shoe.
5. Install hub and brake drum assembly as
directed in "FRONT HUBS AND BEARINGS" (SEC.
3D) of this manual.
6. Adjust brake as previously directed under
"Brake Adjustments."
DUO-SERVO FRONT BRAKE
(Refer to Figure 27)
The Duo-Servo front brakes used on these
vehicles are self-adjusting. Primary linings (shoe
toward front of vehicle) are shorter than second­
ary linings (shoe toward rear of vehicle).
Self-adjusting actuating levers are attached to
the secondary (rear) shoes on all models and oper­
ate when the brakes are applied while the vehicle
is traveling in reverse.
BRAKE SHOE REMOVAL
1. Jack up vehicle and remove wheel.
2. If drum cannot be removed readily, back
off adjustment as described under heading of
"Brake Adjustments."
3. Block up brake pedal so it will not be de­
pressed while drums are removed. Wheel cylinder
boots are recessed in grooves on cylinders, elim ­
inating need for clamps.