A diaphragm pump system includes a diaphragm pump and a pressure regulator. The diaphragm pump has a housing having a pumping chamber containing fluid to be pumped, and a transfer chamber adapted to contain hydraulic fluid. A diaphragm is supported by the housing and at least partially defines a pumping chamber side and a transfer chamber side. A driven plunger slides in a reciprocating motion and forcing hydraulic fluid against the diaphragm. A first valve allows hydraulic fluid into the transfer chamber and a second valve allows hydraulic fluid to be removed from the transfer chamber. A hydraulic fluid reservoir is in fluid communication with the transfer chamber. The pressure regulator includes valving that provides a hydraulic fluid pressure above a pumped fluid inlet feed pressure to maintain a proper amount of hydraulic oil in the transfer chamber.