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GM COACH MAINTENANCE MANUAL
Sec. 4
Page 55
NOTE:
When
necessary
to
replace
supply valve seat, it normally is neces-
sary that the new seat be larger in diam-
eter than the one that was removed to in-
sure a good fit; seats which are .005",
.010", and .015" oversize are available.
Make sure the seat enters straight and is
fully seated.
CAUTION:
Do not damage
sharp edge of seat which is contacted by
supply valve.
5.
Check condition of diaphragm guide damp-
ener ring. When lying on a flat surface, the ends
of the ring should not be in line like a piston
ring; the ring must be twisted so one end is 3/16"
higher than the other.
RELAY VALVE ASSEMBLY (Fig. 9)
1. Lay valve body on bench, bottom side up.
Insert long end of supply valve through valve seat
and position valve on seat.
Place supply valve
spring on valve, then install supply valve cap nut.
Tighten cap nut firmly.
2. Turn assembly over. Install dampener ring
on diaphragm guide, then install guide in body and
over supply valve stem, making sure dampener
ring enters bore in body.
3. Place diaphragm grommet having smallest
diameter in valve body around passage D. Place
diaphragm ring over studs in valve body, with
small bleed hole in ring aligned with passage D.
4.
Place other grommet on diaphragm ring
around bleed hole; then place diaphragms over
studs with large holes in diaphragms aligned with
bleed hole
in
diaphragm ring.
5. Install valve cover on studs with passage
D in cover aligned with passage D in valve body.
Install lock washers and nuts on studs and tighten
evenly and firmly.
6. Install adapter on side of valve body, using
a new adapter gasket.
Tighten adapter attaching
bolts firmly.
BRAKE CHAMBERS
An air brake chamber is mounted at each wheel
and connected to air system as shown in Air Line
Diagram at beginning of this group. Brake cham-
ber used at front brakes is illustrated in figure
10; no boot is used on rear brake chamber, but
a push rod seal washer and spring are used in-
side the non-pressure plate.
Both front and rear
brake chambers have stud type mountings as shown.
The purpose of the brake chambers is to con-
vert the energy of compressed air into the me-
chanical force and motion necessary to operate
the mechanical brake assemblies at each wheel.
Brake chamber consists essentially of a pressure
plate, non-pressure plate, diaphragm, push rod,
AIR BRAKES
and spring.
The yoke on the push rod connects
the push rod to a slack adjuster,
mounted on
brake camshaft.
OPERATION (Fig. 10)
As air pressure enters the brake chamber
behind the diaphragm, the diaphragm forces the
push rod outward, thus applying force to slack
adjuster which rotates brake camshaft, applying
brakes.
When air pressure is released from the
brake chamber, the brake shoe return springs
and the push rod spring return the brake shoes,
brake camshaft, slack adjuster, push rod, and
diaphragm to released position.
Due to the ex-
treme sensitivity of the diaphragm, the push rod
responds to the slightest variation of air pressure
delivered to the brake chamber.
This permits
the driver to apply or release brakes as rapidly
or as gradually as various road or operating con-
ditions warrant.
BRAKE CHAMBER SERVICEABILITY TESTS
1. Operating Test
Apply brakes and see that the brake chamber
push rods move out promptly without binding.
Release brakes and see that they return to re-
leased position promptly without binding.
2.
Leakage Tests
a. With brakes fully applied, coat with soap
suds the bolting flanges holding the diaphragm in
place between the pressure plate and the non-
pressure plate.
No leakage is permissible.
If
leakage is evident, tighten flange bolts. All flange
bolts must be tightened evenly and only sufficiently
Diaphragm
TP 4787
Figure 10-Brake Chamber (Front Shown)
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