When the float is in the "down" position, the pilot port "C" is connected to the "E" port. This vents the diaphragm chamber of the main valve, allowing it to open. The tank fills through the valve until the desired high level is reached; i.e., the float reaches the "up" position. At this point, a connection is made in the pilot from “S" to "C" ports. This applies inlet pressure to the diaphragm chamber of the main valve, causing it to go fully closed.

The above description applies except that the float pilot alternately vents or pressurizes the diaphragm chamber of the three-way auxiliary pilot. In relay fashion, the auxiliary pilot then vents or pressurizes the diaphragm chamber of the main valve.