PC PASSENGER-BRAKE EQUIPMENT 307 



In the oven-eduction position, chambers C and X are con- 

 nected through port /, ports / and y of slide valve 22, and port 

 v of the seat with the chamber G above the service-reservoir 

 charging valve. Also, pressure chamber Z is connected through 

 port /, chamber D, ports c and of the slide valve, and port x 

 of the seat with the chamber K below the service-reservoir 

 charging valve. The pressure in chamber C and, therefore, in 

 chamber G is maintained constant; the pressure in chamber K 

 reduces with the pressure-chamber reduction during an over- 

 reduction, thus insuring that the service-reservoir charging 

 valve will be held down on its seat. 



The service reservoir, the pressure of which is maintained 

 about equal to chamber-C pressure by the pressure-maintaining 

 feature of the application portion, is connected through port g 

 with chamber H. Any slight leakage from the application 

 chamber in this position of the control valve will be supplied 

 from the service reservoir past the packing ring of the service- 

 reservoir charging valve that separates chambers H and G. 

 The capacity of the service reservoir is relatively large when 

 compared with the capacity of the application chamber; there- 

 fore, the pressure in the reservoir will be higher than that 

 in chamber G when a leak develops. Reservoir air will thus 

 leak past the piston-packing ring that separates chambers H 

 and G and prevent any material drop in chamber-C and appli- 

 cation-chamber pressure. Maintaining chamber-C pressure 

 in this manner practically eliminates the possibility of the 

 brakes gradually leaking off, due to application-chamber leak- 

 age, because the pressure-maintaining feature of the control 

 valve will automatically maintain brake-cylinder pressure equal 

 to chamber-C pressure. 



Overreduction Lap Position. When an overreduction is 

 made, the piston 20 moves to overreduction position. This 

 connects chamber D with chamber F, so that the pressure in 

 chambers D, E, and Z gradually reduces by the air discharging 

 into chamber F. When chamber- D pressure becomes enough 

 less than chamber-A pressure for the latter to overcome the 

 factional resistance of piston 20, the piston and the graduating 

 valve 2^ will be moved back to oven-eduction lap position; 

 that is, until the shoulder of the equalizing-piston stem strikes 



