GMC 4000 Series Service Manual page 237

Sec. 5B
Page 230
AIR BRAKES
NOTE OF CAUTION: Air will be ex­
hausted out the line that was connected to
the parking port, if the push-pull control
valve is operated. If this line is not r e ­
moved, it should be disconnected in such a
way that it will not whip and cause damage
as the air exhausts.
4. As a safety precaution, the service system
should also be drained.
5. Disconnect air brake hose at actuator lock
port.
6. Remove yoke pin cotter pin and knock out
yoke pin.
7. Remove mounting nuts, then actuator.
INSTALLATION
IMPORTANT: DD3 brake actuators must be
installed with the drain slot pointing down and
towards the center line of the vehicle.
1. Mount actuator to mounting bracket and
tighten securely.
2. Fasten actuator push rod yoke to slack
adjuster with yoke pin. Lock yoke pin with cotter
pin. The angle formed by the push rod and slack
adjuster arm should be greater than 90°.
3. Connect air brake hoses to actuator taking
precautions that the proper hose is installed in the
correct port, and hoses are positioned for proper
movement during coach operation.
4. Adjust brakes.
5. Build up air pressure in system and test
operation of brakes.
6. Test for leaks.
INVERSION VALVE DESCRIPTION
The inversion valve is used in combination
with the DD3 brake actuators and push-pull control
valve in a parking and/or emergency system . When
the push-pull control valve is operated, the in­
version valve operates permitting air in the park­
ing reservoir to apply the brakes. The inversion
valve also operates automatically when air p res­
sure drops to a predetermined pressure (40 psi).
The inversion valve employs a 1-1/4" diam­
eter hole mounting. The valve serves as a manifold
having a total of six (6) 1/4" pipe ports: three (3)
control ports, two (2) delivery ports and one (1)
supply port. These ports are identified. The con­
trol ports are marked "C," supply "S" and the
delivery ports are marked "D." The exhaust port
is protected by a diaphragm.
INVERSION VALVE OPERATION
With no air pressure in the system , the in­
version valve inlet valve is open and its exhaust
G M C S E R V I C E M A N U A L
is closed. On initial build-up, as air enters the
parking reservoir to which the inversion valve
supply port is connected, it will pass by the open
inlet and out the delivery ports. When system air
pressure reaches between 50 and 60 psi and the
push-pull control valve is pushed in, air will pass
into the inversion valve from the push-pull control
valve. This air flows in one control port and exerts
a force on the inversion valve piston. At a p res­
sure between 60 and 70 psi, the piston moves
against the resistance of the two (2) piston springs.
The piston exhaust seat moves away from the inlet
and exhaust valve, opening the exhaust passage.
The inlet valve spring and supply air in the inlet
valve will cause it to seat. Air at the inversion
valve delivery will now exhaust from the valve
exhaust port.
PARKING OR EMERGENCY
is exhausted through the push-pull control valve
exhaust when it is operated. The piston springs
will then cause the piston to move and seat on the
inlet and exhaust valve closing the exhaust p ass­
age. The inlet valve is moved off its seat by the
piston, so that supply air from the parking re ser­
voir will pass by the open inlet and out delivery
ports.
in on the push-pull control valve, air passes into
the inversion valve control port. The piston moves
away from the inlet and exhaust valve, opening the
exhaust passage through the piston. The inlet valve
closes and air in the delivery ports will exhaust
through the center of the piston stem and out the
inversion valve exhaust port.
PREVENTIVE MAINTENANCE
year or after 50,000 miles the inversion valve be
disassembled, cleaned and lubricated. Lubricate
with a barium base grease (or equivalent).
parts which show signs of wear or damage should
also be replaced.
OPERATING A N D LEAKAGE CHECKS
OPERATING
cut-out pressure and the brakes released, operate
by pulling out the push-pull control valve button
to apply the brakes. Note that the brakes apply.
valve to release the brake application and note
that the delivered air from the inversion valve is
exhausted out its exhaust port. The operation of
the push-pull control valve will not necessarily
The air at the inversion valve control ports
When the application is released by pushing
The manufacturer recommends that every
Rubber parts should be replaced and any other
INVERSION VALVE
With the air brake system built up to governor
Operate by pushing in the push-pull control