SM07-014-0
Page 3
DESCRIPTION
The relay valve functions as a relay station to speed
delivers air to the brake actuators in response to
up the application, modulation, and release of the
the signals received from the parking brake control
valve or the dual brake control valve.
service brakes. It can be considered to be a remote
mounted, air controlled brake valve that releases or
THEORY OF OPERATION
BALANCED - RELAY VALVE NOT DELI VER-
The rapid reaction of the relay valve to changes in
ING AND NOT EXHAUSTING AIR.
control pressures is in part due to the relatively small
volume of air required between the valve cover and
The air pressure being delivered by the open inlet
the relay piston. The area of the relay piston and
valve also is effective on the bottom area of the relay
the pressure of the quick release also contribute
piston. When this air pressure beneath the piston
greatly to the fast application and release of the air
reaches that being delivered above, the piston moves
actuators.
up slightly and the inlet spring returns the inlet valve
APPLYING - RELAY VALVE DELIVERING
line pressure balances the relay valve delivery pres-
AIR PRESSURE.
sure.
When the dual brake valve is actuated, air pressure
RELEASING - RELAY VALVE EXHAUSTING
is delivered to the service port of the relay valve.
DELIVERY PRESSURE.
Air pressure delivered to the service port enters the
small cavity above the piston and moves the piston
When air pressure is released from the servce port
down. The exhaust seat moves down with the
and air pressure in the cavity above the relay piston is
piston and seats on the inner or exhaust portion of
exhausted, air pressure beneath the piston lifts the
the inlet and exhaust valve, sealing off the exhaust
relay piston and the exhaust seat moves away from
passage. At the same time, the outer or inlet portion
the exhaust valve, opening the exhaust passage.
of the inlet and exhaust valve moves off its seat,
With the exhaust passage open, the air pressure in
permitting supply air to flow from the reservoir, past
the air actuators exhausts out the exhaust port to
the open inlet valve and into the air actuator.
the atmosphere.
