groove “i”, thus closing communication between the auxiliary reservoir and the brake pipe, preventing a back-flow of air from the auxiliary to the brake pipe, and at the same time moving the graduating valve 7, opening the service port “Z” in the slide valve.  The continued movement of the piston will move the slide valve until the service port “Z” registers with the brake cylinder port “r” in the valve seat, thus creating a communication between the auxiliary reservoir and the brake cylinder.  Air will now flow from the auxiliary to the brake cylinder until the pressure on the auxiliary side of the piston 4 becomes slightly less than in the brake pipe, when the piston and the graduating valve 7 will move back just far enough to close the service port “Z”, thus closing communication between the auxiliary reservoir and the brake cylinder.  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”.  This permits the air in chamber “Y” to flow through port “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 chamber “a”, the check valve 15 is raised and allows brake pipe air to flow past the check valve and through the ports above mentioned to the brake cylinder.

[Illustration:  Fig. 14.  Quick Service Position.]

The size of these ports are so proportioned that the flow of air from the brake pipe to the top of the emergency piston 8, is not sufficient to force the latter downward and thus cause an emergency application, but at the same time takes enough air from the brake pipe to cause a local reduction of brake pipe pressure at that point, thus assisting the brake valve in increasing the rapidity with which the brake pipe reduction travels through the train.  The triple valve is now said to be in “Quick Service” position. (See Fig. 14.)

153.  Q. Will the triple valve move to quick service position whenever a gradual reduction brake pipe reduction is made?

A. No; with short trains, the brake pipe volume being comparatively small, will reduce more rapidly for a certain reduction at the brake valve than with a long train.  Therefore, with a short train, the brake pipe pressure reducing more quickly, the triple piston and its valves will move to “full service” position, as shown in Fig. 15.  In this position the quick service port “y” is closed, so that no air flows from the brake pipe to the brake cylinder.  Thus, when the brake pipe reduction is sufficiently rapid, there is no need for this quick service reduction, and the triple valve automatically cuts out this feature of the valve when not required.