A gas pressure driven fluid pump comprising a pump tank having a liquid inlet and a liquid outlet. A float, carried within the interior of the pump tank, is operable to move between a low level position and a high level position. A snap-acting valve is operatively connected to the float. The snap acting valve has a rotatable port member that moves so as to switch to exhaust porting when the float falls to the low level position. The port member further switches to motive porting when the float rises to the high level position. As a result, fluid filling the pump tank causes the float to rise from the low level position to the high level position during exhaust porting. Fluid exiting the pump tank causes the float to fall from the high level position to the low level position due to introduction of motive gas during motive porting.