184 TRIPLE VALVES 



port is blanked by the slide valve so that no brake-pipe air 

 can pass to the brake cylinder through the feed-port. The 

 local reduction of brake-pipe pressure at each triple valve 

 is thus prevented because the reduction is being made as 

 fast as desirable at the brake valve, and any local reduction 

 will cause undesired quick action of the brakes. However, 

 the brakes will apply promptly because the service port is 

 fully open and auxiliary pressure reduces at the same rate as 

 brakepipe pressure. 



Lap Position. The lap position assumed, by the triple valve, 

 from quick-service position is different from the lap position 

 assumed from full-service position. This is due to the fact that 

 the slide valve remains stationary and is not moved when the 

 triple piston moves the graduating valve to lap the service 

 ports. The triple valve is held in service position as long 

 as the brake-pipe pressure continues to reduce. When it 

 ceases, auxiliary-reservoir air still flows into the brake cylin- 

 der until the auxiliary pressure is reduced below brake-pipe 

 pressure sufficiently to cause the triple piston to be moved 

 toward release position and the shoulder of the stem comes 

 in contact with the slide valve. The difference in pressure 

 necessary to move the piston and graduating valve is not 

 sufficient to overcome the friction of the slide valve, so that 

 any further movement of the piston is stopped by the slide 

 valve. 



When the piston starts to lap position from quick-service 

 position, the parts come to rest in quick-service lap position. 

 In this position the graduating valve 7 closes the top ends of 

 the graduating port and the port leading to the top of the 

 emergency piston 8, so no more air can pass to the brake 

 cylinder either from the auxiliary reservoir or from the brake 

 pipe. 



If the triple valve is in full-service position when the reduc- 

 tion of brake-pipe air at the brake valve ceases, it will assume 

 lap position in the manner just explained, but the triple piston 

 will be assisted in its movement to lap position by the gradu- 

 ating spring 22, which was slightly compressed, and the piston 

 will be stopped in full-service lap position, instead of in quick- 

 service lap position. 



