Chevrolet 70 1969 Series, Shop Manual

Page: 231 / 913

BRAKES 5B-38
PUSH TO SUPPLY
TRAILER AIR
4
'
PULL FOR
EMERGENCY
Figure ?4— Emergency Control Valve Decal (Conv. 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 E M E R G E N C Y situations.
N O T E : THIS S Y S T E M IS N O T D E ­
SIG N ED T O BE U S E D AS A PARKING
B R A KE U N D E R A N Y C IR C U M S T A N C E S .
There are several different model control
valves used on these vehicles but all 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 pushed in to charge the
trailer emergency line and release the trailer
brakes.
This same type valve is also used with the
D D3 brake system. For information relative to the
function of this valve with the DD3 system, see
procedures given under that heading in this sec­
tion. If a truck has B O T H the DD3 system and
trailer brake controls, there will be T W O 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 complete
assemblies. However on any given vehicle, it is
necessary to use a breakaway valve of the same
manufacturer with each control valve; you can
N O T 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 cab cowl 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 14.
E M E R G E N C Y C O N T R O L
V A L V E R E P L A C E M E N T
Removal
1. Drain air pressure from brake system.
2. Disconnect air lines.
3. Tilt Cab - Remove screws from plate
which holds valve to bracket at left of steering
column. Remove valve.
Conventional Cab - Disconnect knob and
push rod assembly from piston stem. Remove
screws which fasten valve to cowl. Remove valve
from engine compartment side of cowl.
Installation
1. Position valve at mounting bracket (tilt cab)
or cowl (conventional) and attach air lines.
2. Secure valve with screws. 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 operation of trailer emergency brake.
TRACTOR PROTECTION
(BREAKAW AY) VALVE
The tractor protection valve, used in conjunc­
tion with the trailer emergency air supply control
valve, controls operation of the trailer brakes in
E M E R G E N C Y 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 models
(see fig. 11).
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 loss of pressure in the event of pressure loss
in the trailer system.
There are two optional valves used on con­
ventional cab models, one Bendix-Westinghouse and
one Midland-Ross. They are interchangeable as
complete assemblies but each must be used with a
control valve made by the same Manufacturer. You
can N O T use a B-W breakaway valve with an M-R
control valve, and vice versa. The Bendix-West­
inghouse valve has two mounting bolt holes, while
the Midland-Ross valve has three mounting bolt
holes.
B R E A K A W A Y V A L V E R E P L A C E M E N T
Removal
1. Block vehicle wheels.
2. Drain air pressure from brake system.
3. Disconnect air lines.
4. Remove mounting bolts and valve.
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK SH O P M A N U A L