GMC 4000 Series Service Manual page 230

Installation
1. Position valve at mounting bracket (tilt cab)
or cowl (conventional) and attach air lines.
2. Secure valve with screw s. Install plate
where used.
3. On conventional cab models connect knob
and push rod to valve stem.
4. Start engine and build up air pressure to
operating level.
5. Check for air leaks.
6. Check the operation of trailer emergency
brakes.
TRACTOR PROTECTION
(BREAKAWAY) VALVE
The tractor protection valve, used in conjunc­
tion with the trailer emergency air supply control
valve, controls operation of the trailer brakes in
EMERGENCY situations.
This valve is located on the inner side of the
cab back panel on tilt cab models and on the out­
side of the cab back panel on conventional cab
models (fig. 15).
Both service and emergency trailer brake
lines are routed through this valve, and its basic
function is to protect the tractor air brake system
from lo ss of pressure in the event of pressure loss
in the trailer system .
There are two optional valves used on conven­
tional cab models, one Bendix-Westinghouse and
one Midland-Ross. They are interchangeable as
complete assem blies but each must be used with a
control valve made by the same Manufacturer.
You can NOT use a B-W breakaway valve with an
M-R control valve, and vice versa. The Bendix-
Westinghouse valve has two mounting bolt holes,
while the Midland-Ross valve has three mounting
bolt holes.
BREAKAWAY VALVE REPLACEMENT
Removal
1. Block vehicle wheels.
2. Drain air pressure from brake system .
3. Disconnect air lines.
4. Remove mounting bolts and valve.
Installation
1. Position valve on back of cab and install
mounting bolts.
2. Connect air lines.
3. Start engine and build up air pressure to
operating level.
4. Check for leaks.
5. Check operation of brakes.
G M C S E R V I C E M A N U A L
SERVICEABILITY TESTS
Operating Tests
haust air pressure from breakaway valve control
line. Disconnect trailer emergency line from
emergency outlet port at breakaway valve and
connect an air pressure test gauge to emergency
outlet port.
system . Low air pressure buzzer should stop
operating at 58 to 65 psi. With air pressure in
operating range, push emergency air valve knob
in. Test gauge should show full air system p res­
sure. Then pull knob out; test gauge should drop
to zero.
operating range, then stop engine. Push emergency
air valve knob in. Make a ser ie s of brake applica­
tions until low air buzzer sounds. Slightly open
drain cock in the dry air tank to obtain a slow
(approx. 10 psi per min.) pressure drop in the
tractor brake system . When truck air system
drops to 45 psi (approx.), the emergency air valve
knob should automatically come out and pressure
on test gauge should rapidly drop to zero.
Leakage Tests
tem to operating range. Push in emergency air
valve knob to charge trailer brake system . Use
soap suds to coat exhaust ports of emergency air
valve and breakaway valve.
ively, install new piston O-rings. When breakaway
valve leaks excessively, install new O-rings and
plunger seal.
light switch is used in brake system s on all con­
ventional models equipped with trailer brake con­
trols. Check valve is used to connect both the foot
brake application valve and the trailer brake hand
control valve to the trailer brake service line.
Check valve is installed in delivery port in brake
application valve. Air line from trailer brake hand
control valve connects to other end of check valve.
Trailer brake service line and truck stop light
switch are connected to the two side outlet ports.
CHECK VALVE OPERATION
brake valve, air pressure from the foot brake
valve forces the shuttle valve over against the in­
let from the hand control valve and flows out the
1. Pull out emergency air valve knob to ex­
2. Start engine and build up air pressure in
3. Again build up air pressure in system to
1. Build up air pressure in truck brake s y s ­
2. When emergency air valve leaks ex c ess­
DOUBLE CHECK VALVE AND
STOP LIGHT SWITCH
A combination double check valve and stop
When brake application is made by the foot
Sec. 5B
Page 223
AIR BRAKES