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Sec. 4
Page 48
GM COACH MAINTENANCE MANUAL
AIR BRAKES
SERVICEABILITY TESTS
1. Operating Test
Exhaust air pressure from system and install
a test gauge in air brake system, preferably in
the air line leading from air tank to the pressure
regulating valve. Disconnect one of the windshield
wiper air lines at the wiper motor.
Build up pres-
sure in system and note pressure when the valve
permits air to pass to atmosphere.
If
pressure
varies 5 pounds from the original setting (65 lbs.),
the valve requires adjustment.
2.
Leakage Test
With air line disconnected at bottom of valve
and pressure in air brake system below the valve
setting (65 lbs.), coat the bottom opening of valve
with soap suds.
Leakage is caused by dirt on the
valve seat or by an excessively worn valve.
ADJUSTING SET PRESSURE (Fig. 5)
The pressure at which the valve is unseated
is controlled by the adjusting screw (1). Setting
maybe increased or decreased by turning screw.
1. Back off lock nut
(2),
then turn screw
(1)
clockwise to increase pressure, or counterclock-
wise to decrease pressure.
2. Tighten lock nut (2) when correct adjust-
ment is obtained.
REPLACEMENT
1.
Removal
Exhaust air from system and disconnect air
lines.
Remove mounting bolts and remove unit
from vehicle.
2. Installation
Position unit and install mounting bolts.
Con-
nect air lines and test valve as previously directed
under "Serviceability Tests."
TP 4276
Figure 6-Auxiliary Air System Check Valve
DISASSEMBLY (Fig.
5)
Remove four screws
(8)
attaching cover
(3)
to
body (10) and remove cover. Remove spring (5),
spring seat (4), and damper (6) from cover and
lift diaphragm and valve assembly
(7)
off body.
INSPECTION {Fig. 5)
Clean all parts thoroughly, using a suitable
cleaning fluid.
Examine diaphragm for cracks or
wear.
If
either the valve or diaphragm are worn
or damaged, a new diaphragm and valve assembly
should be installed. Inspect valve seat in body and
valve seat assembly.
ASSEMBLY {Fig. 5)
Place diaphragm and valve assembly (7) on
body, with valve seated in valve seat in body. In-
stall spring seat (4), spring (5), and damper (6)
in cover (3) and position cover on body {10). In-
stall four screws (8) through cover and diaphragm
into body and tighten firmly.
Adjust set pressure
as previously directed under "Adjusting Set Pres-
sure."
AUXILIARY AIR SYSTEM
CHECK VALVE
Auxiliary air system check valve (fig. 6), con-
sisting of a valve body, valve, and valve cap, is
connected into air lines at pressure regulating
valve as shown in Air Line Diagram at beginning
of this group. This valve is a one-way valve, per-
mitting air pressure to flow through it in one di-
rection only.
Check valve prevents air pressure
flowing into the auxiliary air system without pass-
ing through the pressure regulating valve; however,
it permits air pressure in the auxiliary air system
to return to the main air brake system when the
pressure in the main air brake system is reduced.
Whenever check valve is removed, it is essential
that it be reinstalled to permit air flow in direc-
tion indicated by arrow on Air Line Diagram.
BRAKE APPLICATION VAL VE
Brake application valve (fig. 7) is a treadle
type D-1 brake valve mounted underneath the toe-
board on the left side of the vehicle. Brake tread-
le and roller assembly is mounted on top of the
toeboard. Application valve is connected into air
lines as shown in Air Line Diagram at beginning
of this group.
APPLICATION VALVE OPERATION (Fig. 7)
1.
Application
As the driver depresses the brake treadle,
pressure is transmitted through the treadle roller,
valve cap, and spring cage to the pressure regu-
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