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Sec. 4
Page 54
GM COACH MAINTENANCE MANUAL
AIR BRAKES
plication valve enters the cavity above the rubber-
ized diaphragm. Since this cavity is comparatively
small and therefore subject to quick changes in
air pressure, the action of the valve in changing
its delivered pressures is very rapid.
1. Applying
As air pressure from the application valve en-
ters cavity A above the diaphragm, the pressure
developed depresses the diaphragm.
The dia-
phragm seals the exhaust port (cavity B) beneath
its outer edge and its center is deflected, forcing
the diaphragm guide down against the supply valve.
Further movement of the diaphragm forces the
supply valve off its seat and air pressure from
the rear air tank flows through the valve into cav-
ity C below the diaphragm and out the ports to the
brake chambers.
2. Holding
As soon as air pressure in cavity
A
above the
diaphragm stops increasing, the pressure below
the diaphragm balances the pressure above it by
means of the by-pass port (passage D).
This bal-
ance of pressures permits the center of the dia-
phragm to rise, removing pressure from the top
of the supply valve. This permits the supply valve
spring to close the supply valve, and the air pres-
sure above the diaphragm keeps the outer edge of
the diaphragm sealing the exhaust port (cavity B).
Thus the valve is in its holding position, maintain-
ing the same pressure in the brake chambers as
the application valve is delivering to it.
A
rise in
application pressure in cavity
A
causes the same
action until an equal pressure is established in
the brake chambers.
3. Releasing
When the application pressure in cavity
A
above
the diaphragm is reduced, the brake chamber pres-
sure in cavity C below the diaphragm forces the
diaphragm up. This opens the exhaust port under
the outer edge of the diaphragm (cavity B), per-
mitting pressure in brake chambers (cavity C) to
exhaust until a lower balanced pressure is reached.
If
the application pressure in cavity A is released
entirely, the relay valve also releases all air pres-
sure from the brake chambers, fully releasing the
brakes.
RELAY VALVE SERVICEABILITY TESTS
1. Operating Test
With the air brake system fully charged, apply
brakes and make sure rear wheel brakes apply
promptly.
Release brakes and make sure air
pressure is quickly exhausted from the exhaust
port of the relay valve.
2.
Leakage Tests
a. With brakes released, cover exhaust port
with soap suds. Leakage in excess of 1-inch bubble
in one second is not permissible.
Leakage is
caused by supply valve not seating properly.
b.
With
brakes applied, cover exhaust port
with soap suds.
Leakage in excess of a 1-inch
bubble in one second is not permissible.
Leakage
is caused by supply valve not seating properly.
c.
If
leakage is caused by diaphragm, both
diaphragm and diaphragm seat should be wiped
clean with gasoline.
If
leakage is caused by supply
valve, valve must be removed and cleaned, or re-
placed with a new part.
RELAY VALVE REPLACEMENT
1. Removal
Exhaust air pressure from system. Discon-
nect air lines from valve, remove mounting bolts,
then remove valve assembly from vehicle. Re-
move stop light switch and tee fitting from top of
valve.
2. Installation
Install tee fitting and stop light switch on top
of valve. Mount valve on frame and tighten mount-
ing bolts firmly.
Connect air lines to valve.
Build
up air pressure in system, then test valve as pre-
viously directed under "Serviceability Tests."
RELAY VALVE DISASSEMBLY (Fig. 9)
1. Remove four bolts and lock washers attach-
ing adapter to valve body. Remove adapter and
gasket.
2. Remove nuts and lock washers from six
studs attaching valve cover to valve body. Remove
cover, diaphragms, diaphragm ring, diaphragm
grommets (at passage D), and diaphragm guide
from valve body. Remove dampener ring from
diaphragm guide.
3. Remove supply valve cap nut from bottom
of valve body. Remove supply valve spring and
supply valve from valve body.
INSPECTION
1. Thoroughly clean all parts in a suitable
cleaning fluid.
2. Examine diaphragms for cracking, stretch-
ing, or deterioration. Replace
if
not in good con-
dition.
3. Examine diaphragm seat at top of valve
body. Seat must be smooth and free from scratch-
es or corrosion.
If
only slightly scratched or
corroded, seat may be repaired by lapping on a
flat surface covered with fine aluminum oxide
abrasive cloth.
4. Examine supply valve and supply valve seat
in body.
If
valve or seat are scratched or worn,
replace with new parts.
To replace seat, press
old seat out of body and press new seat into place,
using new grommet under seat.
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