PC PASSENGER-BRAKE EQUIPMENT 303 



'moves with a less differential pressure, however, owing to the 

 fact that the chamber F at the small end of the piston 20 is 

 open to the atmosphere in release position, thus reducing the 

 area that chamber-!? pressure acts on. A greater reduction 

 in brake-pipe pressure, therefore, is necessary to move piston 

 20 than to move piston 3, so that during a brake-pipe reduction 

 piston 3 moves first. There is a small amount of space between 

 the graduating valve and the release piston, and considerably 

 more between the release slide valve and the release piston. 

 When sufficient brake-pipe reduction is made to overcome the 

 friction of the piston 3, the piston moves forwards, and when 

 it strikes the graduating valve it moves that valve forwards, 

 until, finally, it strikes against and moves the slide valve to 

 the preliminary service position, and the parts assume this 

 position only momentarily on their way to service position. 

 In this position, the piston 3 has moved past the feed-groove 

 s' and has come to rest against the release graduating sleeve 14, 

 as shown. 



In moving the slide valve, port /, leading from the applica- 

 tion chamber to the application-chamber exhaust, is closed. 

 The reduction-limiting chamber F, the service cylinder, the 

 emergency cylinder, and the chambers O and M are all still 

 open to the atmosphere. The connection between chamber F 

 and the atmosphere is now closed, and chamber F is connected 

 through port * and port t of the release slide valve with cham- 

 ber E, and thence through port h, check-valve 16b, and port /, 

 with the pressure chamber Z, thus charging chamber F to pres- 

 sure-chamber pressure and equalizing the pressures on the two 

 faces of the small piston. 



Secondary Service-Application Position. The instant that 

 the pressures on the two faces of the small piston of the equal- 

 izing valve are equalized, the pressure in chamber D exerts a 

 force on piston 20 that is greater than that of the pressure 

 in chamber A ; hence the equalizing valve is moved forwards 

 toward service position. During this movement it momen- 

 tarily assumes secondary service position. In this position, the 

 shoulder on the end of the piston stem is just against the slide 

 valve 22 ; also, port e of the slide valve registers with port c and 

 the graduating valve uncovers port c in the top of the slide valve, 



