GMC 4000 Series Service Manual page 223

Sec. 5B
Page 216
AIR BRAKES
1 Body
2 Ball Seat
3 Ball Valve
4 Exhaust Port
5 Spring
Figure 12— Safety Valve
PRESSURE PROTECTION VALVE
The pressure protection valve is used on some
models. It is mounted in a delivery port of the ap­
plication valve. On some series trucks it may be
used with optional equipment (air shift, differential
lock, etc.). In the optional application, it is located
in the outlet air line of air tank between air tank
and optional equipment.
The function of the valve is to close the air
lines to horns, wipers, transmission shift, differ­
ential lock, etc., when the pressure in the main air
system falls below 65 psi (- 5 lbs.). Thus, in the
event of pressure loss to 65 psi, there still will be
sufficient pressure left to apply service or em er­
gency brakes and stop the vehicle.
REPLACEMENT
Removal
1. Block vehicle wheels.
2. Exhaust air pressure from system .
3. Disconnect air lines to valve.
4. Remove valve from port of application
valve (or air tank line).
AIR TANK
MOISTURE
EJECTOR VALVE
DRAIN EXHAUST
VALVE
Figure 13— M oisture Ejector V a lve Installed (Typical)
G M C S E R V I C E M A N U A L
6 Adjusting Screw
7 Stem
8 Lock Nut
9 Spring Cage
FRAME SIDE RAIL
CAB STEP
SUPPORT
LINE TO
WHEELS
PORT
T-2320
Installation
1. Install valve in delivery port of application
valve (or air tank line).
2. Connect air lines to valve.
3. Build up air pressure in system and check
for leaks.
4. Drain air pressure in system below 65 psi
and check to determine if valve has shut off supply
to applicable units.
QUICK RELEASE VALVE
Quick release valve is used as standard equip­
ment at both the front and rear brakes on some
models, and at front brakes only on other models.
The purpose of the quick release valve is to re­
duce the time required to release the brakes by
hastening the exhaust of air pressure from the
brake chambers. The valve consists of a body
cover and diaphragm so arranged as to permit air
pressure to flow through the valve in one direction.
When application pressure is reduced, the air
pressure which has passed through the valve is
permitted to escape through the exhaust port.
SERVICEABILITY TESTS
1. Operating Test
Apply brakes and observe that when brakes
are released, air pressure is exhausted freely
through the exhaust port of the valve. Be sure the
exhaust port is not restricted in any way.
2. Leakage Test
With brakes applied, coat the exhaust port with
soap suds to check leakage. Leakage is caused
either by dirt on exhaust seat, by a worn diaphragm,
or by a damaged exhaust seat on valve cover.
QUICK RELEASE VALVE REPLACEMENT
Removal
Disconnect air lines from valve. Remove two
bolts attaching valve to crossmember or axle
bracket and remove valve assembly.
Installation
Mount valve on crossmember or axle bracket
and tighten mounting bolts firm ly. Connect air
lines to valve, referring to "Air Lines" section
for torque specifications for air line fittings. Build
up air pressure in system , then test valve as di­
rected under "Serviceability Tests."
MOISTURE EJECTOR VALVE
DESCRIPTION AND OPERATION
The moisture ejector valve is mounted on a
bracket attached to the cab step support close to
the wet air tank. Moisture is ejected each time the
brakes are released. Air pressure lifts actuator