A. Until the main reservoir pressure in chamber "a" under the diaphragm
becomes slightly less than the adjustment of the regulating spring 19,
when the diaphragm 20 will move down, seating the pin valve, shutting
off the flow of air from chamber "a" to chamber "b". The air entrapped
above the governor piston will escape to the atmosphere through the
relief port "c"; this will allow the governor piston to raise, unseating
the steam valve 5, again allowing steam to pass through the governor to
the compressor.
20. Q. Is the maximum pressure head cut out in any position of the
automatic brake valve?
A. No; as the air that operates this head comes direct from the main
reservoir, therefore is not controlled by the brake valve.
21. Q. Is the excess pressure head cut out in any position of the brake
valve?
A. Yes; as the air that operates this head comes through the automatic
brake valve, and when the handle is moved beyond holding position, the
port in the rotary valve seat, through which the air flows to chamber
"d" is closed, thereby cutting out this head, leaving the compressor
under the control of the maximum pressure head.