Abstract: A pump which has a piston (30) which is reciprocatingly moved vertically inside a tubular housing (32) to extract fluid from the bottom of a well (29). The piston (30) is attached to a motor (12) for driving it and has a horizontally extending base plate (50) with a central opening (52) provided with a plug (64) functioning as a check valve. As the fluid is pumped, the plug (64) is raised and the fluid flows into a high pressure reservoir (35) above the base plate (50). When the pressure inside the reservoir (35) exceeds a specified level, the base plate (50) is moved downward against an upward biasing force. This opens an annular port (51) exposing bleed ducts (56) and allows some of the fluid in the reservoir (35) to flow back into low pressure chamber (37) located below the base plate (50).