118 ENGINEER'S BRAKE VALVES 



supply the air, but cannot attain a pressure above that for 

 which the feed-valve is adjusted. Cavity k in the rotary 

 valve connects ports c and g in the valve seat, so that chamber D 

 and the equalizing reservoir charge uniformly with the brake 

 pipe, keeping the pressures on the two sides of the equalizing 

 piston equal. Port s in the rotary valve registers with port p 

 in the valve seat, permitting main-reservoir pressure, which 

 is present at all times above the rotary valve, to pass to the 

 excess-pressure head of the pump governor. Port h in the 

 rotary valve registers with port I in the seat, connecting the 

 application-chamber pipe to the exhaust cavity ex. 



Service Position. The service position gives a gradual 

 reduction of brake-pipe pressure to cause a service applica- 

 tion. Port h in the rotary valve registers with port 5 in the 

 valve seat, allowing air from chamber D and the equalizing 

 reservoir to escape to the atmosphere through cavities o in 

 the rotary valve and exhaust cavity ex in the valve seat. 

 Port e is restricted so as to make the pressure in the equalizing 

 reservoir and chamber D fall gradually. The fall of pressure 

 in chamber D allows the brake-pipe pressure under the equali- 

 zing piston to raise it, and unseat the discharge valve, allowing 

 brake-pipe air to flow to the atmosphere. When the pressure 

 in chamber D is reduced the desired amount, the handle is 

 moved to the lap position, thus stopping any further reduc- 

 tion in that chamber. Air will continue to discharge from the 

 brake pipe until its pressure has fallen to an amount a trifle 

 less than that retained in chamber D, permitting the pressure 

 in this chamber to force the piston downwards and stop the 

 discharge of brake-pipe air. It will be seen, therefore, that the 

 amount of reduction in the equalizing reservoir determines that 

 in the brake pipe, regardless of the length of the train. 



Lap Position. The lap position is used while holding the 

 brakes applied after a service application until it is desired 

 either to make a further brake-pipe reduction, or to release 

 them; also to prevent loss of main-reservoir pressure or the 

 release of the brake in the event of a burst hose, a break in 

 two, or the opening of the conductor's valve. Lap position 

 is also used on all engines in a train that are not controlling 

 the train brakes, as, with the handle in this position, port h 



