3-12. AIR BRAKE SYSTEM MAINTENANCE (CONT)
b. Service Brake Treadle and Valve and Declutch Treadle and Valve (Cont).
THEORY OF OPERATION (SHEET 2 OF 2)
The treadle and valve has a valve assembly for inlet and exhaust. In the bottom of
the valve assembly there is a diaphragm which prevents foreign material from enter-
ing the brake valve through the exhaust port.
Each treadle and valve has a lower chamber and an upper chamber. Air supply from the
air reservoir is always available to the lower chamber of each treadle and valve.
Components of the air system which need constant access to air supply are connected
to ports in the lower chambers. Air supply form the air reservoir is available to
the upper chamber of each treadle and valve only when the pedal is pushed.
When pedal is pressed, force is applied through rubber spring to piston. The piston
moves down. The piston stem makes contact with valve for inlet and exhaust. When the
stem, which is the exhaust seat, makes contact with the valve, the exhaust port from
the upper chamber is closed. As the piston continues to move down, the stem pushes
the valve off the inlet seat, opening the inlet port to the upper chamber. Air sup-
ply from the air reservoir flows through inlet seat and into upper chamber. From up-
per chamber, the air supply flows to brake actuators and to other components connec-
ted to upper chamber.
If low or medium force is applied to pedal, and pedal is held in this position, air
pressure in the upper chamber will soon be equal to mechanical force on the top of
the piston. The rubber spring will let the piston move up until the valve makes con-
tact with the inlet seat, stopping air flow into the upper chamber. The stem of the
piston will continue to keep the exhaust port closed so that air pressure to the
brake actuators stay constant.
If maximum force is applied to pedal, piston will be pushed down completely. The
stem will hold the valve away from the inlet seat. With the piston pushed down com-
pletely, the rubber spring cannot be compressed enough to let the valve make contact
with the inlet seat. Air supply from the air reservoir will continue to be available
to the brake actuators until some of the force is released from the pedal.
When all force is released from the pedal, the spring and the air pressure below the
piston push up the piston. As the piston moves up, the valve makes contact with the
inlet seat, stopping air flow into the upper chamber. The spring continues to push
up the piston. When the stem moves out of contact with the valve, the exhaust port
from the upper chamber opens. Compressed air in the brake actuators flows back
through the air lines and out the exhaust port to the atmosphere.