GMC 4000 Series Service Manual page 229

Sec. 5B
Page 222
AIR BRAKES
T railer Brake Hand
Control Valve
Air Lines to
Application Valve
Application Valve
Stop Light Switch and
Double Check Valve
Figure 16— Trailer Brake Hand Control Valve
Installed— Conventional M odels
TRAILER EMERGENCY AIR SUPPLY
CONTROL VALVES
The trailer emergency supply control valve,
working in conjunction with the tractor protection
(breakaway) valve, controls the operation of the
trailer brakes in EMERGENCY situations.
NOTE: THIS SYSTEM IS NOT DE­
SIGNED TO BE USED AS A PARKING
BRAKE UNDER ANY CIRCUMSTANCES.
G M C S E R V I C E M A N U A L
5 Line to Tractor
Protection Valve
6 Brake Pedal
7 Steering Column
Bracket
8 Steering Column
T-2195
PUSH TO SUPPLY
TRAILER AIR
PULL FOR
EMERGENCY
Figure 17— Emergency Control Valve Decal
There are two different model control valves
used on these vehicles but both operate on the same
principle. They are spring-loaded valves which
will automatically act when pressure in the truck
brake system falls below 45 psi.
Whenever the emergency air valve is actuated,
either manually or automatically, truck air system
pressure must be built up above 45 psi and valve
knob must be manually held in to charge the trailer
emergency line and release the trailer brakes.
This same type valve is also used with the
DD3 brake system which is optional equipment
available on tilt cab models. For information r e l­
ative to the function of this valve with the DD3
system , see procedures given under that heading
in this section. If a truck has BOTH the DD3 s y s ­
tem and trailer brake controls, there will be TWO
control valves and each will be properly identified.
There are two different valves used on con­
ventional cab models, one Bendix-Westinghouse
and one Midland-Ross. These two valves are inter­
changeable on conventional cab models as com­
plete assem blies. However on any given vehicle,
it is necessary to use a breakaway valve of the
same manufacturer with each control valve; you
can NOT use a B-W control valve with an M-R
break-away valve, and vice versa.
On conventional models this valve is located
in the engine compartment on the firewall and is
connected to the knob by a push rod. The knob is
located in the instrument panel in the first hole on
the left side and is identified by a decal on the
panel which is illustrated in figure 17.
EMERGENCY CONTROL VALVE
REPLACEMENT
Removal
1. Drain air pressure from brake system.
2. Disconnect air lines.
3. Steel Tilt Cab - Remove screws from plate
which holds the valve to bracket at left of steering
column. Remove valve.
Conventional Cab - Disconnect knob and
push rod assembly from piston stem. Remove
screw s which fasten valve to cowl. Remove valve
from engine compartment side of cowl.
T- 29 70