Chevrolet 70 1969 Series, Shop Manual

Page: 232 / 913

BRAKES SB-39
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.
SE R V IC E A B IL ITY T E ST S
Operating Tests
1. Pull out emergency air valve knob to ex­
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.
2. Start engine and build up air pressure in
system. Low air pressure buzzer should stop
operating at 58 to 65 psi. With air pressure in oper­
ating range, push emergency air valve knob in.
Test gauge should show full air system pressure.
Then pull knob out; test gauge should drop to zero.
3. Again build up air pressure in system to
operating range, then stop engine. Push em er­
gency air valve knob in. Make a series of brake
applications until low air buzzer sounds. Slightly
open drain cock in the dry air tank to obtain a
slow (approx. 10 psi per minute) pressure drop
in the tractor brake system. When truck air sys­
tem drops to 45 psi (approx.), the emergency air
valve knob should automatically come out and pres­
sure on test gauge should rapidly drop to zero.
Leakage Tests
1. Build up air pressure in truck brake sys­
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.
2. When emergency air valve leaks excess­
ively, install new piston O-rings. When breakaway
valve leaks excessively, install new O-rings and
plunger seal.
DOUBLE CHECK VALVE AND
STOP LIGHT SWITCH
A combination double check valve and stop
light switch is used in brake systems equipped with
trailer brake controls. Check valve is used to con­
nect 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.
C H E C K V A L V E O P E R A T IO N
When brake application is made by the foot
brake valve, air pressure from the foot brake
valve forces the shuttle valve over against the
inlet from the hand control valve and flows out the
holes in the valve guide into the trailer brake
service line. When foot brakes are released and
trailer brakes are applied by means of the hand
control valve, air pressure from the hand control
valve forces the shuttle valve over against the
inlet from the foot brake valve and flows out
through the holes in the valve guide into the trailer
brake service line.
C H E C K V A L V E S E R V IC E A B IL IT Y T E ST S
1. Operating Tests
a. Apply truck brakes and note that brakes
apply promptly on both the truck and the trailer.
Release truck brakes and note that brakes on truck
and trailer both release promptly.
b. Move hand control valve lever to applied
position and note that brakes apply only on the
trailer. Move control valve lever to released posi­
tion and note that trailer brakes release promptly.
2. Leakage Tests
a. Apply truck brakes and check for leakage
at hand control valve exhaust port, using soap suds.
b. Apply trailer brakes only with hand control
valve and check for leakage at truck brake ap­
plication valve exhaust port, using soap suds.
c. No leakage is permissible in either of these
tests. If there is any leakage, replace the shuttle
valve.
C H E C K 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 from valve.
4. Screw valve out of brake application valve.
Installation
1. Screw valve into brake application valve.
2. Connect air lines.
3. Start engine and build up air pressure to
operating level.
4. Check for leaks.
5. Check operation of brakes and stop lights.
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK SH O P M A N U A L