306 PC PASSENGER-BRAKE EQUIPMENT 



pressure by the application portion, as follows: Any reduc- 

 tion in brake-cylinder pressure reduces chamber-O pressure and 

 causes chamber-C pressure to force piston 78 backwards and 

 open the port in the application valve. Air from the service 

 reservoir, therefore, flows through chamber N into chamber O 

 and the brake cylinder until chamber-O pressure is enough 

 greater than chamber-C pressure to overcome the frictional 

 resistance of the piston 78, when the application valve closes 

 and cuts off the flow of air to the brake cylinder. The pressure- 

 maintaining feature of the control valve is the same as that of 

 the ET distributing valve. As will be noted, both chamber F 

 and the emergency-brake cylinder are open to the atmosphere. 



Overreduction Position. The pressures in chambers D and 

 E cannot be reduced below the pressure of equalization of the 

 pressure chamber and the application chamber, which is 86 Ib. 

 from a brake-pipe pressure of 110 Ib. and 54 Ib. from a brake- 

 pipe pressure of 70 Ib. If the brake-pipe pressure is reduced 

 below the pressure of equalization that is, if an overreduction 

 is made the equalizing piston will be moved by chamber-D 

 pressure beyond its service position to the overreduction posi- 

 tion. In this position, the equalizing piston 20 compresses the 

 graduating spring 80 and bottoms against the equalizing-cylin- 

 der cap gasket 27, Release piston 8 remains in service position 

 owing to the higher resistance of the graduating spring 14, 

 which is stronger than the spring 80. 



In moving forwards into overreduction position, slide valve 

 82 is moved so as to close, with port m of the seat, the port I 

 leading to the application chamber and to port I of the slide 

 valve. Port m leads to chamber F, so that on an overreduction 

 the air from the pressure chamber flows into the overreduction 

 chamber F instead of into chamber and the application 

 chamber. The pressure in chamber C is thus held constant 

 at the pressure at which the pressure chamber and the appli- 

 cation chamber equalized; hence, the service-brake-cylinder 

 pressure is limited to this amount and maintained equal to it. 

 The reduction-limiting chamber F is of such size that it 

 will equalize with the pressure chamber at about 60 Ib. from 

 a pressure-chamber pressure of 86 Ib., or at about 35 Ib. from 

 a 70-lb. pressure-chamber pressure. 



