GMC 4000 Series Service Manual page 231

Sec. 5B
Page 224
AIR BRAKES
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 in­
let from the foot brake valve and flows out through
the holes in the valve guide into the trailer brake
service line.
CHECK VALVE SERVICEABILITY TESTS
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
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 18. This
combination perm its full brake valve delivery p res­
sure to the front brakes when on dry roads, or at
the option of the driver, lim its 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 (handle or knob) is
mounted on instrument panel within easy reach of
the driver. On tilt cab models, a plate showing
"DRY ROAD" and "SLIPPERY ROAD" positions is
mounted on the dash panel and a flip-type lever is
used to select the desired position. On all other
models, there is a knob on the dash panel marked
"FRT. BRAKE LIMIT." When pushed in the valve
is in "DRY ROAD" position. Pull knob out for "SLIP­
PERY ROAD" position. On conventional cab models
the valve is located inside the cab. On "cowl"
models the valve is inside the engine compartment.
In both cases, the valve is mounted on a bracket
fastened to the firewall with two bolts.
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. 18). 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
G M C S E R V I C E M A N U A L
FRONT BRAKE LIMITING VALVE A N D
TW O -W AY CONTROL VALVE
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 appli­
cation valve exhaust port, using soap suds.
c. No leakage is perm issible in either of these
tests. If there is any leakage, replace the shuttle
valve.
CHECK VALVE REPLACEMENT
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.
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.
SERVICEABILITY TESTS
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 or knob of the two-way
valve in the "DRY ROAD" position and apply the
truck brakes. Both test gauges should read the
same. Place the handle of the two-way valve in the
"SLIPPERY ROAD" 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 or knob of the two-way
valve in "DRY ROAD" position and with the truck