Abstract: A diaphragm pump includes upper and lower pump bodies clamping a diaphragm therebetween which separates a driving fluid chamber in the upper body from a pump chamber in the lower body, each body having gripping ridges surrounding the chamber; a driving fluid reservoir in the upper pump body; an outlet check valve having a seat, a closure member and a spring to urge its closure; drive means for alternately pressure loading and unloading the driving fluid chamber; a diaphragm having a central body portion connected to an annular clamping portion by annular webbing; a protruding annular rib inside of the ridges, and sized to bite into the diaphragm before the ridges when the bodies are clamped together; means for adjusting the size of an opening between the driving fluid reservoir and the driving fluid chamber; means for adjusting the distance that the closure member can move away from the seat; a piston assembly including piston received within a cylinder having an upwardly facing sealing surface and adapted to