GMC 4000 Series Service Manual page 222

bolts should be checked for looseness at regular
intervals and tightened if necessary. Air tank may
be cleaned inside using steam or hot water. If
corrosion or other damage has weakened the tank,
it must be replaced.
AIR PRESSURE CHECK VALVE
There are several different kinds of check
valves used on vehicles covered by this manual.
Check valves located as separate units in the air
lines (single check valve, double check valve, and
double check valve with stop light switch) are to be
serviced and repaired as covered in the Air Brake
Control Units Overhaul Manual X-5B-01.
Check valve prevents loss of air pressure
from second air tank in the event of leakage in the
first tank or in the air com pressor discharge line.
Arrow on valve body indicates direction of air
flow through valve.
The rubber valve seat should be replaced if
there is any evidence of deterioration or hardening
Valve spring should be replaced if weakened by
rust or corrosion. Valve disc should be perfectly
smooth and free of rust or corrosion. When in­
stalling check valve, make sure it is installed to
perm it air flow from first tank into second tank as
indicated by the arrow on valve body.
A fourth type check valve, which is an integral
part of the divided air tank assembly, is to be
serviced and repaired as detailed in the following
procedures (refer to fig. 10). This valve is built
into the tank at the point where the "wet" tank por­
tion is divided from the "dry" tank portion (refer
to fig. 11).
All check valves, regardless of type, should
be removed, disassembled, and cleaned or replaced
every six months, 50,000 m iles, or 1800 hours
(whichever occurs first).
Figure 1 0 -A ir Tank C heck Valve
G M C S E R V I C E M A N U A L
DIVIDED TANK CHECK VALVE SERVICE
(Refer to Fig. 10)
both valve spring and valve seat spring and parts
will "fly off" if not removed carefully.
ation, or swollen condition. Discard any bad parts.
Install seat and O-ring assembly, valve, and springs.
nut and thread cap nut into tank. Tighten cap nut.
mine effectiveness of check valve by opening drain
cock on "wet" portion of tank and note pressure
retention in "dry" portion of tank.
inate the possibility of air pressure building up in
the system beyond a safe maximum in the event
of failure of the air com pressor governor.
OPERATION
exceed 145 to 155 psi, air pressure forces ball
valve off seat, permitting air to escape through
exhaust port to atmosphere. After pressure bleeds
down, spring forces ball back onto the seat.
MAINTENANCE
using soap suds at exhaust port, with 90 pounds
pressure in tank. Leakage should not exceed a 3-
inch bubble in 3 seconds. Once a year, safety valve
should be disassembled, cleaned with kerosene,
and reset to blow off at 145 to 155 psi.
ADJUSTMENT (Fig. 12)
Turn clockwise to increase pressure or counter­
clockwise to decrease pressure.
INLET
PORT
1. Drain air from all reservoirs.
2. Remove cap nut carefully. Nut com presses
3. Remove springs, valve, and valve seat.
4. Inspect all parts for cracking, deterior­
5. Clean valve seat area.
6. Reassemble, using new parts as necessary.
7. Compress springs by pushing down on cap
8. Build up air pressure in system and deter­
SAFETY VALVE
A safety valve is installed in air tank to elim ­
When pressure in the air tank is built up to
Check safety valve periodically for leakage,
1. Loosen lock nut.
2. Adjust pressure by turning adjusting screw.
3. Tighten lock nut.
CHECK V ALVE
"W ET" TA N K
^ DRAIN PORT TANK DIVIDER
Figure 11— D ivid e d Air Tank
Sec. 5B
Page 215
AIR BRAKES
DRY" TANK
OUTLET
PORT