GMC 4000 Series Service Manual page 228

gauge should be checked regularly with an accurate
test gauge. If pressure reading varies 4 pounds or
more, replace gauge with a recalibrated unit.
LOW AIR PRESSURE SWITCH
Low air pressure switch is used with a buzzer
as a low air alarm. Buzzer is also used as a part
of engine alarm system . The switch is installed in
a delivery port of the application valve or in a
fitting which is installed in a delivery port of the
application valve. Switch is adjusted and sealed by
INDEPENDENT TRAILER
BRAKE APPLICATION
On vehicles equipped with trailer brake hand
control valve, trailer brakes may be applied with­
out applying the truck brakes. When hand control
valve lever is moved to applying position, air
pressure passes through the hand control valve
and double check valve into trailer brake service
line, applying the trailer brakes. When hand con­
trol valve lever is returned to released position,
air pressure in the trailer brake service line is
exhausted from exhaust port of the control valve,
releasing the trailer brakes. Any desired degree
of trailer brake application is obtained by regulat­
ing the position of the hand control valve lever.
I.C.C. BRAKE SYSTEM EQUIPMENT
Some vehicles are equipped with special I.C.C.
(Interstate Commerce Commision) brake system
equipment in conjunction with the trailer brake
controls. These controls are a tractor protection
(breakaway) valve and a trailer emergency air
supply control valve. These controls are provided
for the following purposes:
1. They protect the tractor air system from
complete loss of air pressure in the event of a
trailer breakaway or lo ss of air pressure in the
trailer air system .
2. They automatically actuate the trailer brake
emergency relay valve (on trailer) in case the
tractor air system pressure falls below 45 psi.
3. They provide the driver with a manual
means of actuating the trailer brake emergency
relay valve (on trailer).
TRAILER BRAKE HAND
CONTROL VALVES
Three different trailer brake hand control
valves are used on these models. Midland-Ross
G M C S E R V I C E M A N U A L
the manufacturer and is not reparable. Switch has
only one wire terminal, being internally grounded.
trolled switch in an electrical circuit, automatic­
ally controlling a buzzer in the cab. A tell-tale
light is also used. Refer to applicable "Wiring
Diagram" in Manual X-6905 for electrical circuits.
58 to 65 psi. If switch fails, it must be replaced.
When installing switch, do not use sealing com ­
pound on threads, since the switch is grounded
through its mounting and sealing compound will
act as insulation.
TRAILER BRAKE CONTROLS
valve Model N-3973-H is used on all tilt cab
models. The other two hand control valves are
used optionally on conventional cab models. Bendix-
Westinghouse (Model TC-2) and Midland-Ross
(Model N-30060) are completely interchangeable.
column with the operating handle on the right-hand
side (fig. 16). The valve graduates the delivered
air pressure in proportion to the degree the oper­
ating handle is moved. That is the farther the
handle is moved from "OFF" to "ON," the greater
the air pressure delivered to the trailer brakes.
If, at any time during such an application a service
brake application is also made with the foot-con­
trolled valve, whichever valve releases the greater
air pressure will control the brakes.
necessary to hold the handle in the desired position
in order to apply trailer brakes. When handle is
released, it automatically returns to the "OFF"
position. The Bendix-Westinghouse valve (Model
TC-2) and the Midland-Ross valve on tilt cab
Models (N-3973-H) will remain in the placed
position until manually moved. Other than this
operation of all three valves is identical.
HAND CONTROL VALVE REPLACEMENT
Removal
and valve to steering column.
Installation
steering column. Fasten with bolts.
The low air pressure switch is an air-con-
The switch has a nominal pressure setting of
All three valves are mounted on the steering
On Midland-Ross valve Model N-30060 it is
1. Block vehicle wheels.
2. Drain air from brake system.
3. Disconnect air lines from valve.
4. Remove bolts attaching mounting bracket
5. Remove valve and bracket.
1. Position valve and mounting bracket on
2. Connect air lines to valve.
3. Start engine and build up air pressure.
4. Check for leaks and operation of brakes.
Sec. 5B
Page 221
AIR BRAKES