AUTOMATIC BRAKE VALVE (cont)
(d) Equalizing reservoir air pressure is directed through an orifice from the charging passage to the
diaphragm of the regulating valve.
(e) Through port 5, the equalizing reservoir air pressure is directed to the outer side of the relay valve
(3) Brake pipe air pressure is developed as follows:
(a) Equalizing reservoir air pressure on the outer side of the relay valve diaphragm forces the diaphragm
and valve stem inward. Inward movement of the valve stem opens the supply valve seat, and main
reservoir air enters the brake pipe charging passage.
(b) The brake pipe charging pressure flows to the brake pipe cutoff valve, which opens when the charging
air pressure is about 25 psi (172 kPa). Air then flows around the sealed vent valve, into the brake pipe
passage, and out port 1.
(c) Brake pipe charging air pressure is also directed through an orifice to the inner face of the relay valve
diaphragm, where it develops a counteracting force to the equalizing reservoir force on the outer face
of the diaphragm. When these forces are equal, brake pipe charging cuts off.
(d) Brake pipe air pressure is also connected through a passage in the brake valve to spillover check
valve B on the bottom of the handle portion, where it is made available to the spool of the cutoff pilot
valve in passage 1A.
(4) Lap position is maintained as follows:
(a) When the equalizing reservoir charging air pressure in pipe 15 and on the inner face of the regulating
valve diaphragm are equal to the adjustment spring tension of the regulator valve, the valve spool
moves toward the adjustment spring. This moves the supply valve against its seat and stops charging
of the equalizing reservoir. This is self-flapping. If a leak in the equalizing reservoir occurs, the
diaphragm pressure weakens and the regulator valve moves to supply and the equalizing reservoir
pressure is brought back up to make the adjustment spring tension. This is equalizing reservoir
(b) Brake pipe charging continues until pressure on the inner face of the relay valve diaphragm matches
the pressure of equalizing reservoir control pipe 5 on the outer face of the diaphragm, The valve stem
is then drawn back, away from the supply seat, allowing it to close and stop brake pipe charging. The
valve self-laps. If a leak in the brake pipe occurs, the pressure on the inner face of the diaphragm
weakens and the equalizing reservoir pressure on the outer face of the diaphragm moves the relay
valve to supply until the brake pipe pressure once again matches the equalizing reservoir pressure.
This is brake pipe pressure maintaining.