The air distribution system directs the air supply to the right air chamber and therefore, the back side of diaphragm A. The compressed air moves diaphragm A away from the center block toward the liquid chamber.

The opposite diaphragm (diaphragm B) is pulled inward by the shaft connected to the pressurized diaphragm (diaphragm A). Diaphragm B is now on its suction stroke; air behind the diaphragm has been forced out to atmosphere through the exhaust port of the pump. Diaphragm A is currently working against atmospheric pressure.

The movement of diaphragm B toward the center block of the pump creates a vacuum within liquid chamber B. Atmospheric pressure forces fluid into the inlet manifold forcing the inlet valve ball off its seat. Liquid is now free to move past the inlet valve ball (lower left) and fill the liquid chamber.