368 HIGH-SPEED REDUCING VALVE 



As long as the cylinder pressure remains less than 60 Ib. 

 per sq. in., the reducing valve plays no part in an ordinary 

 service application of the brake, the valve remaining in its 

 normal position, with port a blanked. Suppose, that in making 

 a service application, the brake-cylinder pressure should 

 increase above 60 Ib.; the pressure above the piston 4 will be 

 sufficient to compress the regulating spring, and the piston 

 and slide valve will be forced downwards until the base, or 

 largest part of port b, registers with port a. In this position, 

 brake-cylinder air is free to flow to the atmosphere through 

 the exhaust fitting 24 until the pressure is reduced to 60 Ib., 

 when the regulating spring forces the piston and slide valve 

 upwards into their normal positions again. The area of the 

 opening through ports a and b in this position is such that air 

 can discharge from the cylinder as fast as it enters through the 

 service port in the slide valve of the triple. 



In an emergency application of the brake, air enters the 

 brake cylinder from the train pipe and auxiliary reservoir in 

 much greater volume than it could possibly escape through 

 the ports a and b of the reducing valve; hence, piston 4 of 

 the latter is forced downwards the full length of its stroke, 

 and assumes a position such that the apex of the triangular 

 port b registers with port a. In this position, the passage 

 through ports a and b is small and air discharges quite slowly 

 from the cylinder. As the pressure in the cylinder, and conse- 

 quently above piston 4, gradually decreases, due to the dis- 

 charge through ports a and b, the regulating spring gradually 

 raises the piston and slide valve, and, as the slide valve is 

 raised, the opening through ports a and b gradually increases; 

 consequently, the discharge from the cylinder increases accord- 

 ingly until the brake-cylinder pressure is reduced to a safe 

 amount (60 Ib.), when the reducing valve assumes its normal 

 position, covering the opening a so that no more air can 

 escape from the brake cylinder until brakes are released. 



