TM 55-2210-223-34
TO 45A2-2-14-22
4-11.
AUTOMATIC BRAKE VALVE (cont)
TEST (cont)
b.
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
obtained.
(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.
(2) Brake pipe air pressure reduces as follows:
(a) Pressure reduction in the equalizing reservoir will be felt on the outer side of the relay valve
diaphragm. With the inner face of the diaphragm at brake pipe pressure, the pressure difference will
force the diaphragm outward, pulling the valve stem with it.
(b) The valve stem will open the exhaust seat of the relay valve. Brake pipe air pressure will be vented
out relay valve exhaust port, causing a brake pipe pressure reduction at service rate.
(c) Brake pipe air pressure will continue to vent until pressure on the inner face of relay valve diaphragm
and in the brake pipe balances with the pressure of the equalizing reservoir. Relay valve will then
assume lap position.
(d) Equalizing reservoir air pressure can be reduced in steps, by handle movement, in about 2-psi (14
kPa) increments until a pressure reduction of 23 to 26 psi (159 to 179 kPa) is attained at FULL
SERVICE position.
4-49
