A pump comprising a valve housing and a spool slidably positioned within the valve housing. The spool has a supply face, a first end face, and a second end face, and is slidable between a first position wherein pressurized supply air is supplied to the supply face and the first end face, the first end face having a greater surface area than the supply face, a second position wherein the supply air is supplied to the supply face and is blocked from both the first and second end faces, and a third position wherein the supply air is supplied to the supply face and the second end face.