GMC 4000 Series Service Manual page 212

NO BOOST — HARD PEDAL
PROBABLE CAUSE
1 . Bent, broken obstructed tube. Collapsed hose.
2. Stuck check valve.
3. Air inlet blocked.
4. Air valve stuck.
5. Faulty diaphragm.
6. Faulty piston seal.
7. Leaks internally.
8. Leaking vacuum tank.
SLOW BRAKE PEDAL RETURN
PROBABLE CAUSE
1. Excessive seal friction in power unit.
2. Faulty valve action.
3. Broken return spring.
BRAKES GRABBY
PROBABLE CAUSE
1. Broken valve spring.
2. Sticking vacuum valve.
3. Reaction diaphragm leakage.
G M C S E R V I C E M A N U A L
1 . Replace defective parts.
2. Replace valve.
3. Replace filter. Open passages.
4. Disassem ble unit - - clean - - replace defective
parts.
5. Replace diaphragm.
6. Replace seal.
7. Rebuild.
8. Repair tank.
1. Rebuild unit.
2. Rebuild unit.
3. Replace spring.
1. Rebuild unit.
2. Clean and lubricate.
3. Rebuild unit.
Sec. 5A
Page 205
HYDRAULIC BRAKES
REMEDY
REMEDY
REMEDY