DuPont Delrin diaphragm made of Buna-N (Nitril), reinforced with Dacron valves are duck bill style (better for debris) made of Buna-N (Nitril) fasteners made of stainless steel these pumps are used in applications where there is less then 2 feet of suction lift since it is the return spring that pulls the water into the pump.
For example: foot operated bilge pumps (boats, kayaks, canoes, etc.) water supply in trailers flush pump in portable toilets water display in museums and science centres stronger springs are available for higher suction lifts,