TM 5-3810-306-20
Applying - Normal Operation of No. 1 Circuit Portion
When the No. 1 inlet valve is moved off its seat, air is permitted to pass through the bleed passage and enters the relay
piston cavity. The air pressure moves the relay piston, which contains the exhaust seat and closes the No. 2 exhaust
valve. As the No. 2 exhaust valve closes, the No. 2 inlet valve is moved off its seat allowing the No. 2 air to flow out the
No. 2 delivery port. Because of the small volume of air required to move the relay piston, action of the No. 2 circuit
portion of the valve is almost simultaneous with the No. 1 circuit portion.
Applying Loss of Air in the No. 2 Circuit
Should air be lost in the No. 2 circuit, the No. 1 circuit portion will continue to function as described under Normal
Operation of No. 1 Circuit Portion.
Applying Loss of Air in the No. 1 Circuit
Should air be lost in the NO. 1 circuit, the function will be as follows. As the brake treadle is depressed and no air is
present in the No. 1 circuit supply and delivery ports, the No. 1 piston will mechanically move the No. 2 exhaust valve
open the No. 2 inlet valve and allow air to flow out the No. 2 delivery port.
Balanced No. 1 Circuit Portion
When the No. 1 delivery pressure acting on the piston equals the mechanical force of the brake pedal application, the No.
1 piston will move and the No. 1 inlet valve will close, stopping the further flow of air from the No. 1 supply line through
the valve. The exhaust valve remains closed preventing any escape of air through the exhaust port.
Balanced No. 2 Circuit Portion
When the air pressure on the No. 2 side of the relay piston approached that being delivered on the No. 1 side of the relay
piston, the relay piston moves closing the No. 2 inlet valve and stopping further flow of air from the supply line through the
valve. The exhaust remains closed as the No. 2 delivery pressure balances the No. 1 delivery pressure.
When applications in the graduating range are made, a balanced position in the No. 1 portion is reached as the air
pressure on the delivery side of the No. 1 piston equals the effort exerted by the operator’s foot on the treadle. A
balanced position in the No. 2 position is reached when air pressure on the No. 2 side of the relay piston closely
approaches the air pressure on the No. 1 side of the relay piston.
When the brake treadle is fully depressed, both the No. 1 and No. 2 inlet valves remain open and full reservoir pressure
is delivered to the actuators.
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