When the dual brake valve is actuated, air pressure is delivered to the service port of the relay valve. Air pressure
delivered to the service port enters the small cavity above the piston and moves the piston down. The exhaust seat moves
down with the piston and seats on the inner or exhaust portion of the inlet and exhaust valve, sealing off the exhaust
passage. At the same time, the outer or inlet portion of the inlet and exhaust valve moves off its seat, permitting supply air
to flow from the reservoir, past the open inlet valve and into the actuator.
The air pressure being delivered by the open inlet valve also is effective on the bottom area of the relay piston. When this
air pressure beneath the piston reaches that being delivered above, the piston moves up slightly and the inlet spring
returns the inlet valve to its seat. The exhaust remains closed as the service line pressure balances the relay valve
When air pressure is released from the service port and air pressure in the cavity above the relay piston is exhausted, air
pressure beneath the piston lifts the relay piston and the exhaust seat moves away from the exhaust valve, opening the
exhaust passage. With the exhaust passage open, the air pressure in the air actuators exhausts out the exhaust port to
Foot Brake Valve
The foot brake valve is mounted to the cab floor. It is a treadle-operated type brake valve with two separate supply and
delivery circuits. The valve provides the driver with a graduated control for applying and releasing the vehicle brakes.
The circuits in the foot brake valve are identified as follows. The No. 1 circuit portion is that portion of the valve between
the spring seat which contacts the plunger and the relay piston. The No. 2 circuit portion is that portion between the relay-
piston and the exhaust cavity.
The No. 1 circuit portion of the valve is similar in operation to standard single-circuit air brake valve and under normal
operating conditions the No. 2 circuit portion is similar in operation to a relay valve.
Both the No. 1 and No. 2 circuit portions of the dual brake valve use a common exhaust protected by an exhaust
For explaining the operation of the foot brake valve, the valve is divided into a No. 1 circuit portion and a No. 2 circuit
Applying - Normal Operation of No. 2 Circuit Portion
When the brake treadle is depressed, the plunger exerts force on the spring seat, rubber graduating spring, and No. 1
piston. The No. 1 piston which contains the exhaust valve seat, closes the No. 1 exhaust valve. As the exhaust valve
closes, the No. 1 inlet valve is moved off its seat allowing No. 1 air to flow out the No. 1 delivery port.