Chevrolet 70 1969 Series, Shop Manual

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BRAKES 5B-40
FRONT BRAKE LIMITING VALVE AND
TW O-W AY CONTROL VALVE
A combination limiting and quick release valve
and a two-way control valve are used in combin­
ation on some vehicles as shown in figure 15. This
combination permits full brake valve delivery pres­
sure to the front brakes when on dry roads, or at
the option of the driver, limits the pressure to the
front brakes to 50 percent of the brake valve de­
livery pressure when on slippery roads.
The two-way control valve is mounted on in­
strument panel within easy reach of the driver.
The limiting quick release valve is mounted on
frame crossmember near the front brake cham­
bers. One air line from brake application valve
is connected to the inlet port of the two-way valve
and another connects to the brake valve port at top
of limiting quick release valve (fig. 15). Another
air line connects the side delivery port of the two-
way valve to the port opposite the mounting pad of
the limiting quick release valve. The two other
side ports of the limiting quick release valve are
connected to the front brake chambers.
The limiting quick release valve, besides pro­
viding for a 50 percent reduction of front wheel
brake pressure, also serves as a quick release
valve when brakes are released.
SER V IC E A B ILIT Y T EST S
1. Operating Tests
a. Connect an air pressure test gauge into
the air line leading to the rear brake quick release
or relay valve; disconnect air line at valve and
connect gauge to line if no other convenient con­
necting point is available. Disconnect one front
brake chamber line from port at side of limiting
quick release valve and connect another test gauge
to this port.
b. Place the handle of the two-way valve in
the " D R Y R O A D " position and apply the truck
brakes. Both test gauges should read the same.
Place the handle of the two-way valve in the "SLIP­
P E R Y R O A D " position and apply the truck brakes.
The test gauge at the limiting quick release valve
should read approximately one-half the amount
shown on the test gauge connected to the rear axle
air line.
2. Leakage Tests
a. Place the handle of the two-way valve in
" D R Y R O A D " position and with the truck brakes
applied, coat the exhaust ports of the two-way
valve and limiting quick release valve with soap
suds. If leakage is excessive, valve should be
replaced or repaired.
b. Place the handle of the two-way valve in
the "SLIPPER Y R O A D " position and with the
truck brakes applied, coat the exhaust port of the
two-way valve with soap suds. If leakage is ex­
cessive, valve should be replaced or repaired.
T W O - 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 screws which fasten plate and valve
to instrument panel. Remove plate and valve. For
overhaul, refer to "Differential Control Valve" in
"R E A R A X L E " (SEC. 4A) of this manual.
Installation
1. Position valve in instrument panel and con­
nect air lines.
2.
Fasten valve and plate to instrument panel.
3. Start engine and build up air pressure to
operating level.
4. Check for leaks. Check operation of brakes.
LIM ITIN G V A L V E R E P L A C E M E N T
Removal
1. Block vehicle wheels.
2.
Drain air from brake system.
3. Disconnect air lines.
4. Remove bolts which fasten valve to vehicle.
Remove valve.
Installation
1. Connect air lines.
2.
Position valve on vehicle and fasten with
bolts.
3. Start engine and build up air pressure to
operating level.
4. Check for leaks.
5.
Check operation of brakes.
DD3 BRAKE ACTUATOR SYSTEM
DESCRIPTION
are, the brake actuator (chamber), a push-pull
control valve (for manual parking and emergency
The D D 3 brake actuator system consists of application), and an inversion valve (to direct the
three major units connected by air lines. These flow of air pressure to either parking or locking
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L