TM 55-203
(2) Secondary action. At the same time, the
first movement of the graduating valve connects the two
ports o and q in the slide valve through the cavity v in the
graduating valve, and the movement of the slide valve
brings port o to register with port y in the slide valve seat,
and port q with port t. Consequently, the air in chamber Y
flows through ports y, o, v, q and t, thence around the
emergency piston 8, which fits loosely in its cylinder, to
chamber X and the brake cylinder. When the pressure in
chamber Y has reduced below the brake pipe pressure
remaining in a, the check valve 15 is raised and allows
brake pipe air to flow past the check valve and through the
Figure 8-41. The auxiliary reservoir variable release
ports mentioned above to the brake cylinders. The size of
valve.
these ports is so proportioned that the flow of air from the
brake pipe to the top of emergency piston 8, is not
sufficient to force the latter downward and thus cause an
emergency application, but at the same time enough air is
taken from the brake pipe to cause a definite local
reduction in brake pipe pressure at that point, which is
transmitted in like manner to the next ,triple valve and, in
turn, to the next, thus increasing the rapidity with which the
brake pipe reduction travels through the train.
d. Full-Service Position. With short trains, the brake
pipe volume being comparatively small, the pressure will
reduce more rapidly for a certain reduction at the brake
valve than with long trains. Under such circumstances, it
Figure 8-42. Branch pipe tee.
might be expected that the added reduction at each triple
valve by the quick-service feature would bring about so
rapid a brake pipe reduction as to cause quick action and
an emergency application, when only a light application
was intended. (The emergency application is explained
later. ) But this is automatically prevented by the triple
valve itself.
From figure 8-46 it will be noted that in the quick-service
position, port z in the slide valve and port r in the seat do
not fully register. Nevertheless, the opening is sufficient to
allow the air to flow from the auxiliary reservoir to the brake
cylinder with sufficient rapidity to reduce the pressure in the
Figure 8-43. Sectional view of cutout cock
auxiliary reservoir as fast as the pressure is reducing in the
brake pipe, when the train is of considerable length. If the
brake pipe reduction is more rapid than that of the auxiliary
reservoir, which may be the case on short trains, the
difference in pressure on the two sides of piston 4 soon
becomes sufficient to slightly compress the graduating
spring, and move the slide valve
Figure 8-44. Hose connection
8-47
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