GMC 4000 Series Service Manual page 259

Sec. 5B
Page 252
AIR BRAKES
"FAIL-SAFE" PARKING BRAKE WILL NOT RELEASE
PROBABLE CAUSE
1. Air leak in "Fail-Safe" lines
2. Loose clamp ring or cap
3. Leakage at release bolt
4. Faulty piston O-ring seal
5. Foreign material in "Fail-Safe" chamber
. Improper wedge adjustment
6
7. Corroded "Fail-Safe" spring
"FAIL-SAFE" PARKING BRAKE WILL NOT APPLY
PROBABLE CAUSE
.
1 "Fail-Safe" spring not fully released
. Inoperative parking brake control valve or
2
quick release valve
3. Foreign material in chambers or piston stuck
in cap
4. "Fail-Safe" spring failure
5. Rollers not aligned with plungers or wedge not
mated with diaphragm plate rod
. Brakes out of adjustment
6
AUTOMATIC ADJUSTERS INOPERATIVE
PROBABLE CAUSE
1. Guide installed backwards
2. Guide spring missing or weak
3. Plunger seal failure
4. Adjusting bolt threaded into actuator too tight
5. Plunger guide washer omitted
G M C S E R V I C E M A N U A L
STOPMASTER TROUBLESHOOTING CHART
REMEDY
. Check air system and correct leaks
1
. Tighten and check for air leaks
2
3. Replace piston and seal assembly
4. Replace piston O-ring seal
5. Clean with solvent and lubricate
. Make wedge adjustment
6
7. Replace cap and spring assembly
REMEDY
.
1 Turn release bolt counterclockwise
. Check operation of valves as outlined in
2
Maintenance Manual and replace if necessary
3. D isassem ble and clean with solvent; lubricate
piston and cap
4. Replace cap and spring assembly
5. Remove non-pressure housing and check
installation of wedge
. Adjust brakes; check operation of automatic
6
adjusters
REMEDY
1. Remove plunger guide components and
reassem ble
2. Replace guide spring
3. Replace seal; clean and lubricate actuation
parts
4. Back off bolt 1/4 turn
5. Replace washer