Diaphragm pump with dual spring overfill limiter

A diaphragm pump includes a housing having a pumping chamber containing fluid to be pumped. The pump has a transfer chamber adapted to contain hydraulic fluid, and a hydraulic fluid reservoir in fluid communication with the transfer chamber. The pump housing forms a cylinder with a piston sliding in a reciprocating motion in the cylinder, the piston defining a piston inner chamber. A valve leads to the piston inner chamber with a valve spool slidably mounted in the piston inner chamber to cover the valve in a first position and uncover the valve in a second position. 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 in the piston inner chamber is intermediate the valve spool and the spacer. A second spring is in the piston chamber 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.