TRUCK SERVICE MANUAL
The relay valve (Fig. 1) is used to operate the
brakes on the rear axle of six wheel tractors, trucks or
When the brake valve is applied, air pressure from
long wheelbase trucks.
the brake valve passes into the service port to fill the
small cavity above the diaphragm in the relay valve. This
It is operated by air pressure from the foot brake
pressure acting against diaphragm assembly (12) forces
valve. The air pressure delivered to the brake chambers
the plunger assembly down to close exhaust valve (18)
is the same as the pressure delivered to the relay valve.
and open inlet valve (22) allowing air pressure from the
reservoir port and lower cavity of the relay valve to pass
With a direct air supply from the reservoir, the relay
through the cylinder ports to the brake chambers. At the
acts as a remote brake valve to speed up the application
same time, air pressure from the lower cavity passes
and release of brakes.
through a small orifice to the cavity below the diaphragm.
When this pressure builds up to equal the pressure
at the service port, spring (17) moves the diaphragm
upward, to close inlet valve. The relay valve is now in
Lap position and instantly responsive to any movement
of the brake valve to increase or decrease the brake
When the brake valve is released, air pressure at the
service port exhausts thru the foot brake valve, spring
(17) moves diaphragm assembly (12) to full upward
position. Exhaust valve (18) opens to permit air from the
brake chambers to pass through the hollow section of
the diaphragm and plunger assembly and the exhaust
port of the relay.
1. Remove three capscrews (3), lock washers (2),
and five nuts (15), lock washers(14), and bolts
Fig. Cross Sectional View of Relay Valve
2. Remove cover (7), diaphragm and plunger
Body, Relay Valve
assembly, and spring (17) from body (1).
3. Remove screw (9), retainer (10), and diaphragm
4. Remove diaphragm retainer nut (16), retainer
Not Used Ret.
plate (4), diaphragm (12), from plunger (11).
5. Remove five capscrews (25), lock washers (26)
Diaphragm, Relay Valve
to remove end cap (23).
6. Remove cage assembly (21) and two gaskets
7. To disassembly cage assembly, remove nut
(24), inlet valve (22) from exhaust valve stem
Nut, Valve Stem
(18). Remove exhaust valve (18) and spring