Abstract: My invention is a down hole pump that operates on hydrostatic pressure. The mandrel is a tubular member having perforations therein. The expandable tube made of high quality plastic surrounds the hollow mandrel covering the perforations, thereby forming a means for communicating hydrostatic pressure existing within said hollow mandrel on an upstroke of said pump to expand the expandable tube. There is a plurality of special expandable rings which surround the expandable tube. The expandable tube causes the rings to expand radially with the expandable tube so as to maintain constant contact with the barrel. There is also a plurality of expandable spacers surrounding the expandable tube between each of the expandable rings which have a significantly smaller outside diameter than the expandable rings. There are also means provided to secure the expandable tube, rings and spacers to the mandrel.