TRIPLE VALVES 157 



in front of a port leading to the brake-cylinder pipe connection. 

 This position of the valve is called the service position. 



When the graduating valve is off its seat, there is an open 

 communication between the auxiliary and the brake cylinder 

 and air flows from the auxiliary into the brake cylinder, where 

 the pressure will force out the brake piston and set the brakes. 

 Just as long as the auxiliary pressure is greater than that in 

 the brake pipe, so long will piston 5 be held down and the gradu- 

 ating valve remain unseated; but the auxiliary pressure gradu- 

 ally expands into the brake cylinder, until the pressure in the 

 lower chamber is sufficiently greater than that in the upper 

 chamber to overcome the small friction of the packing ring 6 

 and cause piston 5 to be moved upwards and seat the graduating 

 valve. The pressure on the brake-pipe side of the piston 5 

 still slightly exceeds that in the auxiliary, but not to such an 

 extent as to overcome the additional friction encountered in 

 moving the slide valve 3; the piston therefore stops as soon 

 as the graduating valve has been seated. This is called the 

 lap position of the triple valve. In this position all ports are 

 blanked. The brakes are now partly set; a further brake-pipe 

 reduction will be necessary to apply them harder. 



If another 5-lb. brake-pipe reduction is made, the greater 

 auxiliary pressure again forces down the piston, but in this 

 case the slide valve is already in service position, and it is 

 only necessary to move the piston sufficiently to unseat the 

 graduating valve. This is accomplished by the time the knob 

 touches the graduating stem 8\ and once more, by means of the 

 service port of the slide valve, communication is established 

 between the auxiliary and the brake cylinder. The graduating 

 valve is again seated automatically by the piston 5 when the 

 auxiliary pressure becomes a little less than that in the brake 

 pipe. 



After the slide valve has once been moved down, it remains 

 in service position until the brakes are released. Each reduc- 

 tion of brake-pipe pressure causes the brake to set harder, 

 and these reductions may be continued just as long as the 

 pressure in the auxiliary is greater than that in the brake 

 cylinder. When these pressures become equalized, the brake 

 is fully set, and a further brake-pipe reduction will be a waste 



