GMC 1976 ZEO 6083 Maintenance Manual page 232

POWER CYLINDER TROUBLE DIAGNOSIS CHART
BRAKES FAIL TO RELEASE
Cause
Blocked passage in diaphragm plate
Air valve sticking shut .
Broken piston return spring .
Broken air valve spring .
Tight pedal linkage.
HARD PEDAL
Cause
Broken or damaged hydraulic
brake lines.
Vacuum Failure.
Before checking the power brake system for the source of trouble, refer to
the trouble diagnosis of drum and disc brakes . After these possible causes
have been eliminated, check for the cause as outlined in the following chart :
Defective diaphragm.
Restricted air filter element.
Worn or badly-distorted reaction
disc .
Incorrect reaction disc .
GRABBY BRAKES
(Apparent Off-On Condition)
Cause
Broken or damaged hydraulic
brake lines.
Insufficient fluid in master
cylinder
Defective master cylinder seals .
Cracked master cylinder casting.
Leaks at front disc brake
calipers or rear wheel cylinders
in pipes or connections.
Air in hydraulic system .
Inspect and repair or replace as necessary.
Check for proper lubrication of air valve "O" ring .
Replace.
Replace.
Repair or replace as necessary.
Inspect and replace as necessary .
Check for :
Faulty vacuum check valve or grommet-replace .
Collapsed or damaged vacuum hose-replace .
Plugged or loose vacuum fitting-repair .
Faulty air valve seal or support plate seal-replace .
Damaged control valve-replace.
Bad stud welds on front or rear housing or
head-replace, unless easily repaired .
Replace.
Replace.
Replace reaction disc .
Replace with correct disc .
Correction
Correction
Correction
Inspect and replace as necessary .
BRAKES 5- 1 3
power
Fill reservoirs with approved brake fluid, check
for leaks.
Repair or replace as necessary.
Replace.
Inspect and repair as necessary.
Bleed system .