y a waste of air. With
seventy pounds brake pipe pressure, and eight-inch piston travel, a
twenty-pound reduction will cause equalization at about fifty pounds.

[Illustration: Fig. 16. Lap Position.]

156. Q. Explain the operation of the triple valve in the release of the
brake.

A. To release the brakes and recharge the auxiliary reservoirs, air is
admitted through the brake valve to the brake pipe. This increase of
pressure on the brake pipe side of the triple valve piston 4 above that
on the other side causes the piston and slide valve to move back to
release position, which permits the air in the brake cylinder to flow to
the atmosphere, through the exhaust port of the triple, thus releasing
the brake. At the same time, air from the brake pipe flows through the
feed groove "i" around the triple piston to the auxiliary reservoir,
which is thus recharged. Now the "K" triple valve has two release
positions: =Full Release= and =Retarded Release=. To which of these two
positions the parts will move when the brakes are released, depends upon
how the brake pipe pressure is increased. It is generally understood
that those cars toward the head end of the train, receiving the air
first, will have their brake pipe pressure raised more rapidly than
those in the rear; thus the friction of the brake pipe causes the
pressure to build up more rapidly in the chamber "h" of the triple valve
toward the front end of the train than in those in the rear. As soon as
the pressure is enough greater than the auxiliary reservoir pressure to
overcome the friction of the piston, graduating valve and slide valve,
all three are moved toward the left until the piston stem strikes the
retarding stem 31, which is held in position by the retarding spring 33.
Where the rate of increase of brake pipe pressure is slow, it will be
impossible to raise the pressure in chamber "h" sufficiently to overcome
the tension of the retarding spring 33, and the triple valve will remain
in full release position, as shown in Fig. 13. Brake cylinder air will
now be free to exhaust through port "r", large cavity "n" in the slide
valve and port "p" leading to the atmosphere. If, however, the triple
valve is near the head end of the train, and the brake pipe pressure
builds up more rapidly than the auxiliary can recharge, an excess of
pressure will be obtained in chamber "h" over that in the auxiliary
reservoir, and will cause the piston 4 to compress the retarding spr