GMC 4000 Series Service Manual page 238

complete the release of the brakes. A full 100 psi
service brake application after the push-pull con­
trol valve is pushed in should complete the release.
LEAKAGE
Start leakage checks with system pressure
up to governor cut-out and brakes released. Check
the inversion valve exhaust port for possible leak­
age at (1) the large piston grommet; (2) the piston
stem grommet or; (3) the inlet valve or its seat.
Slight leakage is perm issible. While the inversion
valve is still in this position, the cap nut should
be checked for leakage by the seal ring.
Actuate push-pull control valve by pulling out
on the button to apply the brakes, then check the
inversion valve exhaust port for exhaust valve or
seat leakage.
If the inversion valve does not function as
described or leakage is excessive, it is recom ­
mended that it be repaired or replaced.
INVERSION VALVE REPLACEMENT
(Refer to Figure 20)
REMOVAL
1. Block and hold vehicle by means other than
air brakes.
2. Drain service and parking reservoir air
supply.
3. Disconnect air lines and air brake hoses
from inversion valve.
4. Loosen mounting nut and remove valve.
INSTALLATION
1. Check and clean air lines to valve.
2. Mount valve securely with mounting nut and
lock washer.
3. Connect air lines and air brake hoses.
4. Check for proper operation and leaks.
PUSH-PULL CONTROL VALVE
DESCRIPTION
The push-pull control valve is located in the
dash panel. It is to the right of the steering col­
umn. The valve has a black knob which moves in
and out ("push-pull") for operation. The lettering
on the knob is illustrated in figure 21. A red col­
ored ring on the knob, when exposed, indicates that
parking brake is on. The valve has four ports; two
delivery, one supply and one for connection to the
service brake foot control valve. The push-pull
valve directs the flow of air to the DD3 brake ac­
tuator through the inversion valve. The direction
of flow is controlled within the valve by a combin­
ation inlet and exhaust valve in conjunction with
the position of the knob ("push" for service, -
"pull" for parking).
G M C S E R V I C E M A N U A L
refer to figure 19 and the description given previ­
ously under "Service Brake Operation," "Parking
Operation" and "Emergency Operation."
REMOVAL
air brakes.
is unnecessary to exhaust service air.
trol valve. Mark lines to assure installation in
correct valve ports when replacing valve.
button.
remove valve from dash panel.
INSTALLATION
with mounting nut.
in button with hole in plunger shaft and drive in
spirol pin.
ports making sure lines are connected to correct
ports.
check valve for operation and leaks.
the trailer emergency brakes when tractor em er­
gency push-pull control valve is pulled. It joins
the trailer emergency brake system to the tractor
parking and emergency brake system . It is used
only on vehicles which have trailer brake connec­
tions and air operated emergency brakes (L4000
only).
Figure 2 1 — D D 3 Control Valve Knob
CONTROL VALVE OPERATION
For operation of the push-pull control valve,
CONTROL VALVE REPLACEMENT
1. Secure vehicle by some means other than
2. Exhaust air from parking brake system . It
3. Disconnect air lines from push-pull con­
4. Drive spirol pin out of button and remove
5. Remove mounting nut from valve body and
1. Position valve on dash panel and secure
2. Place button on plunger shaft, line up hole
3. Connect air lines to push-pull control valve
4. Build up vehicle air pressure system , and
SYNCHRO VALVE
The purpose of the Synchro valve is to activate
Sec. 5B
Page 231
AIR BRAKES