GMC 4000 Series Service Manual page 258

AIR PRESSURE ABOVE NORMAL
PROBABLE CAUSE
1. Defective air gauge
2. Governor out of adjustment or defective
3. Restricted line between governor and com­
p ressor
4. Compressor unloader inoperative
QUICK LOSS OF PRESSURE WHEN ENGINE
IS STOPPED
(BRAKES NOT APPLIED)
PROBABLE CAUSE
1. Leaking lines or connections
2. Worn or leaking com pressor exhaust valve or
one-way check valve.
3. Leaking governor
4. Leaking application valve
5. Open or leaking reservoir drain cock
(BRAKES FULLY APPLIED)
1. Leaking brake chamber
2. Leaking application valve
3. Leaking service line
4. Leaking chamber hoses
5. Dirt in two-way check valve. (If equipped with
hand control valve and brakes are applied by
the foot valve, dirt in the two-way check valve
could cause a pressure leak at the hand valve
exhaust port.)
. Defective quick release valve
6
SAFETY VALVE "BLOWS OFF"
PROBABLE CAUSE
1. Safety valve out of adjustment
2. Pressure in system above normal
3. Governor out of adjustment
G M C S E R V I C E M A N U A L
. Replace
1
. Adjust, repair or replace
2
3. Clear line or replace
4. Repair or replace
. Repair or replace
1
. Repair or replace
2
3. Repair or replace
4. Repair or replace
5. Close, repair or replace
. Repair or replace
1
. Repair or replace
2
3. Repair or replace
4. Repair or replace
5. Clean or replace
. Clean or replace diaphragm.
6
. Adjust
1
. See chart on "Air pressure above normal"
2
3. Adjust
Sec. 5B
Page 251
AIR BRAKES
REMEDY
REMEDY
REMEDY