GMC PD-4103 Maintenance Manual page 57

GM COACH MAINTENANCE MANUAL
Sec. 4
Page 49
AIR BRAKES
1
Treadle Stop Screw 13 Diaphragm and Spring
2 Treadle Seat Assembly
3 Treadle Roller 14 Shims
4 Valve Cap 15 Grommet
5 Boot
16 Diaphragm Return
6 Spring Cage Spring
7
Shims
17 Lower Body
8 Grommet
18
Grommet
9 Pressure Regulating 19 Exhaust Valve Seat
Spring 20 Inlet Valve Assembly
10 Plunger
21 Shims
22
~~
23
I
11
Body Bolts 22 Grommet
12 Upper Body 23 Inlet Body
24 Outlet Ports
TP 4270-1
Figure 7-Brake Application Valve
lating spring and diaphragm. As the diaphragm
and plunger moves downward, the exhaust valve
seat contacts the exhaust valve. Continued down-
ward movement of diaphragm and plunger pushes
the inlet valve off its seat. This permits air pres-
sure to flow through the inlet valve and into the
lines leading to the front and rear brakes.
2. Holding
The compensating port (passage A) in the low-
er body casting permits the same air pressure
being delivered to the brakes to enter the cavity
below the diaphragm. When air pressure in cav-
ity below diaphragm balances the mechanical force
on top of the diaphragm, the diaphragm moves up-
ward, permitting the inlet valve to close but not
enough to open the exhaust valve. This prevents
additional pressure from being delivered to the
brakes, and at the same time prevents pressure
already delivered from escaping through the ex-
haust valve and exhaust port, and brakes are held
in applied position. Should the driver depress the
brake treadle still further, exerting additional
force on top of the diaphragm, a corresponding
increase in air pressure being delivered to the
brakes results.
If the driver depresses the treadle to fully
applied position, the pressure regulating spring is
compressed and the spring cage strikes the spring
seat. Under these conditions, the inlet valve is
held open permitting full air system pressure to
pass through the valve to the brakes.
3. Partial Release
If
the driver permits the treadle to partially
return toward its released position, reducing the
mechanical force on top of the diaphragm, the air
pressure below the diaphragm overcomes the me-
chanical force on top of it. When this occurs, the
diaphragm lifts, closing the inlet valve and open-
ing the exhaust valve, permitting air pressure in
cavity below diaphragm to escape through the ex-
haust port until the pressure below the diaphragm
again balances the mechanical force on top of it.
4. Release
When the driver permits the brake treadle to