AIR COMPRESSORS 89 



as to be always in communication with the main reservoir, 

 so that when the excess-pressure head is cut out by the brake 

 valve, or by the main-reservoir cut-out cock, this head will 

 control the pump. When main-reservoir pressure in the cham- 



ber below the diaphragm 36 exceeds the adjustment of spring 19, 

 the diaphragm will raise the diaphragm valve and allow air 

 to flow into the chamber above the governor piston, controlling 

 the pump as just described. The adjustment of spring 19 thus 



