A. When any sudden reduction of brake pipe pressure is made below that
in the auxiliary reservoir, it will be felt in chamber "h" in front of
piston 4 and cause this piston to move to the extreme right, as shown in
Fig. 18. This movement of the parts will open port "t" in the slide
valve seat and allow air from the auxiliary reservoir to flow to the top
of the emergency piston 8, forcing the latter downward and opening
emergency valve 10. The unseating of the emergency valve allows the air
in chamber "Y" to escape to the brake cylinder, thus permitting brake
pipe pressure in chamber "a" to lift the check valve 15 and flow to the
brake cylinder through chambers "Y" and "X", until brake cylinder and
brake pipe pressure nearly equalize, when the check valve is forced to
its seat by the check valve spring 12, preventing the air in the
cylinder from escaping back into the brake pipe again. The emergency
valve and piston will now return to their normal position. At the same
time port "s" in the slide valve registers with port "r" in the slide
valve seat, and allows air from the auxiliary reservoir to flow to the
brake cylinder. This sudden discharge of brake pipe air into the brake
cylinder has the effect on the next triple valve, which in turn vents
brake pipe air that affects the following triple valve and so on
throughout the train.