4-11. AUTOMATIC BRAKE VALVE (cont)
(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.
SERVICE Position. As the handle is moved from RELEASE position toward SERVICE position, the brake pipe
pressure reduction is gradually increased until in FULL SERVICE position a full service brake pipe reduction is
(1) Equalizing reservoir air pressure reduces as follows:
(a) When handle is moved into the SERVICE zone, the regulating valve cam rotates to a lower grind and
allows the regulating valve spool to move toward the handle shaft. This action opens the exhaust seat of
the regulating valve and weakens the adjustment spring tension.
(b) Equalizing reservoir air flows from the equalizing reservoir to the regulating valve exhaust port, through
pipe 15 and the equalizing reservoir cutoff valve in the brake valve. The equalizing reservoir pressure
will be reduced at a controlled rate as the regulating valve vents. Air pressure on the regulating valve
diaphragm will also be reduced at the same rate.
(c) Handle movement into the service zone will also rotate suppression valve cam to move suppression
valve spool inward to an intermediate position. In this position, passage and port 3 air pressure will be
vented through the valve spool. Venting of passage 3 air pressure will result in venting of passage 7,
through cutoff pilot valve. Air pressure holding up the check of the equalizing reservoir cutoff valve will
be removed and the check will seal. Air pressure from the equalizing reservoir can pass toward the
regulating valve to exhaust, allowing graduated exhaust of equalizing reservoir pressure as the brake
valve handle is advanced through the service zone. The equalizing reservoir cutoff valve will not allow
an increase in equalizing reservoir pressure as the brake valve handle is moved toward release until the
handle is returned fully to RELEASE position.