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

Abstract: A pressure unloading valve responsive primarily to the pressure of the outlet passage in a diaphragm pump. The pump includes a driving fluid chamber and a pump chamber separated by a flexible diaphragm. Between the pump chamber and the outlet passage is a check valve allowing fluid flow only from the pump chamber to the outlet passage. The unloader valve is unseated, allowing relief of fluid pressure in the driving fluid chamber, when the pressure in the outlet passage exceeds a first predetermined value or when the pressure in the driving fluid chamber exceeds a second predetermined value. An adjustment knob is provided to vary the first and second predetermined values. An anti-blow-by seal is provided to check fluid seepage from the driving fluid chamber to the sump.