When the water level in reservoir becomes low, the chamber inside of body becomes in the condition of atmospheric pressure as opening pilot valve, piston moves to upward by the water pressure because the lower area of piston is larger than upper side of disc and disc assembled to piston opens at the same time and fluid can flow toward outlet.
CLOSE
When the water level in reservoir reaches to high water level just, pilot
valve will automatically close. In this point, the chamber pressure inside
of body is same with the pressure of inlet by water route through the
chamber and the liquid room of inlet, fluid flowing toward outlet will
close since disc closes valve seat as moving downward by the weight of
piston assembly and the power of spring. At this time, liquid pressure
by the area of valve seat is total power of valve closing.