304 PC PASSENGER-BRAKE EQUIPMENT 



so that there is a momentary connection between the emergency 

 reservoir and chamber D while the slide valve is moving past 

 the secondary service position. The object of this is to charge 

 chamber D from the emergency reservoir an amount sufficient 

 to compensate for the increase in volume in chamber D as the 

 piston 20 moves forwards to service position, and thus prevent 

 a drop in chamber-/) pressure due to the increased volume. 

 Also, momentary connection is made between chamber D and 

 the pressure chamber Z through the groove d in the equal- 

 izing-valve seat, port d in the valve face, the cavity dw in the 

 graduating valve, and ports / in the slide valve and seat. This 

 prevents chamber D from being highly overcharged and main- 

 tains the pressures in chambers D and Z equal. The pressure 

 in chamber E is maintained equal to the pressure in chamber Z 

 through port /, check-valve 16b, and port h as the piston 3 

 moves forwards. In fact, during an application of the brakes, 

 this connection practically makes chambers E and Z but one 

 chamber in volume; that is, during a reduction the connection 

 through the check- valve 16b maintains the pressure equal in the 

 two chambers, so that to reduce the pressure in chamber E, 

 the pressure in chamber Z must be reduced a like amount. 



Service Position. The piston 20 moves forwards from sec- 

 ondary service position to service position, where it is stopped 

 by the equalizing graduating spring 30. In this position, port 

 u connects chambers D and F, thus equalizing the pressures in 

 the two chambers. The pressure chamber has a direct con- 

 nection to chamber D by way of port / and a port through the 

 equalizing slide valve 22. Also, it has an indirect connection 

 with chamber D through the check-valve 16b, port h, chamber 

 E, port t of the slide valve 4, and port i; port i divides, one 

 branch leading to chamber F and the other to a port in the 

 equalizing slide valve. These two paths make provision for a 

 considerable volume of air to flow from the pressure chamber 

 into chamber D. 



Pressure-chamber air, after flowing to chamber D, can pass 

 through port I in the slide valve 22 and port / in the valve 

 seat to chamber C ahead of the application piston 78, and to 

 the application chamber X. The pressure thus admitted 

 into chamber C moves piston 78 backwards to its application 



