Abstract: A well-pressure operated pump having a tubular pump body provided with an interior, annular seating surface to receive a valve member mounted on a rod extending through the seating surface. An air spring in the pump body urges the rod upwardly in response to fluid pressure in the pump body and the height of the seating surface and the position of the valve member on the rod are adjustable for adjusting the well pressure at which the valve member is seated. Seal assemblies mounted on the pump body are adjustable to match the pump to the internal diameter of the well casing and the seal assemblies are provided with retainer rings having flanges that capture end portions of a sealing member of each seal assembly. A brake, having a plate that is spring loaded to overlay a shoulder in the bore of the pump body, slows the rate of descent of the pump through gases in the well. An aspirator formed in the support for the plate holds the plate off the shoulder during passage of the pump through liquids in the well.