A diaphragm pump includes a housing having a pumping chamber containing fluid. The pump has a transfer chamber containing hydraulic fluid, and a hydraulic fluid reservoir in fluid communication with the transfer chamber. The pump housing forms a cylinder with a piston defining a piston inner chamber and sliding in the cylinder. A valve leads to the piston inner chamber with a valve spool slidably mounted in the piston inner chamber to cover the valve and uncover the valve. A diaphragm connects to the valve spool by a plunger. An overfill limiter includes a spacer slidably mounted in the piston inner chamber. A first spring is intermediate the valve spool and the spacer and a second spring is intermediate the end of the piston inner chamber and the spacer, the second spring having a second spring constant greater than the spring constant of the first spring.