Drum Brakes; Description; Brake Lining Replacement - GMC G Service Manual

5A2-10 HYDRAULIC FOUNDATION BRAKES

DESCRIPTION

The drum brake assembly is a duo-servo design. With
this particular design, the force applied by the wheel cylin­
der to the primary shoe forces the leading edge of the shoe
into contact with the rotating drum, causing the shoe to try
to rotate. The shoe, in its attempt to rotate with the drum,
transfers force to the secondary shoe through the star-
wheel adjuster. This causes the secondary shoe's leading
edge to "bite" into the drum and attempt to rotate, just as the
primary shoe does. Since the shoes cannot rotate, they
tend to wedge themselves into the drum. In this way, the
rotating torque augments the braking force applied to the
shoes by the wheel cylinder. Because the shoes are used
for this wedging action, the system is called duo-servo, as
opposed to a single servo design where wheel cylinder
pressure alone is the source of braking force.
The torque from the brake shoes is transferred through
the backing plate to the axle flange. Brake adjustments are
automatic and are made during reverse brake applications.

BRAKE LINING REPLACEMENT

H Remove or Disconnect (Figure 22)
• Raise the vehicle and support with suitable safety
stands.
• Mark the relationship of the wheel to the hub.
1. Wheel and tire assembly. Refer to WHEELS AND
TIRES (SEC. 3E).
CAUTION: See "Caution" on page 5A2-1 of this
section.
2. Drum.
• Mark the relationship of the drum to the axle.
3. Return springs (12 and 13).
4. Shoe guide (7).
5. Hold-down springs (14).
6. Hold-down pins (1).
7. Actuator lever (10) and lever pivot (15).
8. Lever return spring (16).
9. Actuator link (11).
10. Parking brake strut (8).
11. Strut spring (9).
12. Retaining ring (6).
13. Parking brake lever (3) and washer (4).
14. Shoes (5 and 19).

DRUM BRAKES

15. Adjusting screw assembly (17).
16. Adjusting screw spring (18).
I ? | Important
I P I Inspect
• All parts for discoloration due to heat or stress.
Replace if necessary.
• All parts for signs of wear. Replace if necessary.
• Wheel cylinder for signs of leakage. Refer to
"Rebuilding The Wheel Cylinder" in this section.
• Brake drum for scoring and machining tolerance.
Refer to "Servicing The Brake Drum" in this section.
H Install or Connect (Figure 22)
• Lubricate the shoe pads and adjusting screw threads
with a thin coat of white lithium grease.
1. Adjusting screw (17) and adjusting screw spring (18)
to both shoes (5 and 19).
2. Shoe assembly.
3. Parking brake lever (3) and washer (4) into the shoe.
4. Retaining ring (6).
5. Strut spring (9) onto the parking brake strut (8).
6. Parking brake strut (8).
7. Actuator lever (10) and lever pivot (15).
8. Actuator link (11).
9. Lever return spring (16).
10. Hold-down pins (1).
11. Hold-down springs (14).
12. Return springs (12 and 13).
13. Drum.
14. Wheel and tire assembly. Refer to WHEELS AND
TIRES (SEC. 3E).
• Adjust the brakes. Refer to "Brake Adjustment" in this
section.
• Do not interchange right and left adjusting
screws.
• The coils of the spring must not touch the
adjusting screw.
• Align the marks made during disassembly.
• Align the marks made during disassembly.