Chevrolet 70 1969 Series, Shop Manual

Page: 229 / 913

BRAKES 5B-36
Figure 12— Trailer Hose Coupling
six months, or more often under severe operating
conditions.
2. Dum m y Couplings
Dum m y couplings are mounted on rear of cab
and serve as hangers for the trailer hose when
they are disconnected from the trailer. Both hoses
should always be connected to the dummy coup­
lings when not connected to the trailer.
3.
Cut-out Cocks (When Used)
Cut-out cocks are used in the trailer brake
lines on vehicles not equipped with trailer break­
away valve. Purpose of cut-out cocks is to provide
a means of closing off these lines when they are
not being used. The cut-out cock is open when the
handle is at a 90-degree angle to the body, and is
closed when the handle is parallel with the body.
Stops are provided so the handle cannot be turned
beyond its normal open and closed positions.
Cut-out cocks should be tested periodically
for leakage. Connect cut-out cock to source of air
pressure, build up 90 pounds pressure against it,
and cover opening with soap suds. If leakage ex­
ceeds a 1-inch bubble in 3 seconds, cock should be
repaired or replaced. In some instances, leakage
may be caused by dirt on the key. This condition
can be remedied by cleaning the key and seat with
gasoline, then grinding the key to its seat with
grade 400 grinding compound. Thoroughly wash off
all grinding compound and coat key lightly with a
good grade of cup grease before assembling. If the
key or body is badly scored, the complete cut-out
cock should be replaced.
AIR PRESSURE GAUGE
The air pressure gauge in the instrument
panel provides the driver with a means of check­
ing system air pressure. The vehicle should never
be put in motion until pressure reaches 65 pounds.
If pressure reading drops to less than 65 pounds
while vehicle is in motion, vehicle should be stop­
ped and the cause of air loss corrected. Dash 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
the manufacturer and is not reparable. Switch has
only one wire terminal, being internally grounded.
The low air pressure switch is an air-
controlled s w i t c h in an electrical circuit,
automatically controlling a buzzer in the cab. A
tell-tale light is also used. Refer to applicable
Wiring Diagram in Section 12 for electrical
circuits.
The switch has a nominal pressure setting of
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
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 regu­
lating the position of the hand control valve lever.
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L