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 spring 33, and move the triple valve parts to retarded release position as shown in Fig. 17.

157.  Q. What effect has retarded release position of the triple valve on the release of the brakes?

A. In this position of the triple valve, cavity “n” in the slide valve connects port “r” leading to the brake cylinder, with port “p” to the atmosphere, and the brake will release; but as the small “tail port” extension of cavity “n” is over exhaust port “p”, the discharge of air from the brake cylinder is quite slow.

[Illustration:  Fig. 17.  Retarded Release and Charging Position.]

158.  Q. What is the object of delaying the exhaust of the brake cylinder air?

A. In this way, the brakes on the front end of the train require a longer time to release than those on the rear.  This feature is called =retarded release=, and although the triple valves near the locomotive commence to release before those in the rear, yet the exhaust of air from the brake cylinder is sufficiently slow to hold back the release of the brakes at the front end of the train long enough to insure a uniform release of the brakes on the train as a whole.  This permits of releasing the brakes on very long trains at low speeds without danger of damaging train.

[Illustration:  Fig. 18.  Emergency Position.]

159.  Q. What other desirable feature is found in this position of the triple valve?

A. In this position, the back of the piston is in contact with the end of the slide valve bush, and, as these two surfaces are ground to an accurate fit, the piston makes a tight “seal” on the end of the bush except at one point, where a feed groove is cut in the piston to allow air to pass around the end of the slide valve bush into chamber “R” and the auxiliary reservoir.  This feed groove is much smaller than the feed groove “i” in the piston bush, so that when the triple valve piston is in =Retarded Release= position the recharge of the auxiliary reservoir takes place much more slowly than when it is in =Full Release= position, thus permitting a greater volume and pressure of air to flow toward the rear of the train.

160.  Q. Explain the operation of the triple valve in emergency position.