GMC 4000 Series Service Manual page 211

Sec. 5A
Page 204
HYDRAULIC BRAKES
BRAKES CHATTER
PROBABLE CAUSE
1. Incorrect lining to drum clearance.
2. Loose backing plate.
3. Grease, fluid, road dust on lining.
4. Weak or broken retractor spring.
5. Loose wheel bearings.
6. Drums out-of-round.
7. Cocked or distorted shoes.
8. Tapered or barrel-shaped drums.
SHOE CLICK
PROBABLE CAUSE
1. Shoes lift off backing plate and snap back.
2. Hold down springs weak.
3. Shoe bent.
4. Grooves in backing plate pads.
TROUBLE SHOOTING POWER HYDRAULIC BRAKES
NOTE: The same types of brake troubles are encountered with power brakes as
with standard brakes. Before checking the power brake system for
source of trouble, refer to trouble diagnosis of standard hydraulic brakes.
After these possible causes have been eliminated, check for cause as
outlined below.
NOTE: Make the following test before checking hard pedal for the cause. With
the engine stopped, depress the brake pedal several times to eliminate
all vacuum from the system . Apply the brakes, and while holding the foot
pressure on the brake pedal, start the engine. If the unit is operating
correctly, the brake pedal will move forward when the engine vacuum
power is added to the pedal pressure. If this test shows that the power
unit is not operating, the trouble may be one of the following:
BRAKE SYSTEM LOSES FLUID
PROBABLE CAUSE
1. External Leak: Leaking-pipe connections, hose,
wheel cylinders, master cylinder head nut, etc.
2. Internal Leaks: Past secondary seals into
power unit. Check vacuum hose for fluid.
G M C S E R V I C E M A N U A L
1. Readjust to recommended clearances.
2. Tighten securely.
3. Clean or reline.
4. Replace.
5. Readjust.
6. Grind or turn drums in pairs.
7. Straighten or replace.
8. Grind or turn in pairs.
1. Change drums side to side or grind drums
(in pairs).
2. Replace springs.
3. Straighten.
4. Grind and lubricate.
(Vacuum A ssist Units)
1. Clean parts. Tighten. Replace defective parts.
2. Rebuild master cylinder.
REMEDY
REMEDY
REMEDY