A hydraulically driven pump includes a diaphragm, a piston, a transfer
chamber, a fluid reservoir, and a spool member. The transfer chamber is
defined between the diaphragm and piston and is filled with a hydraulic
fluid. The fluid reservoir is in fluid communication with the transfer
chamber via at least one valve. The spool member is configured to control
fluid flow between the transfer chamber and the fluid reservoir. The
spool member is movable to open and close an opening into the at least
one valve only when an overfill condition or an underfill condition
exists in the transfer chamber.