Abstract: A pump for pumping fluid down a well bore in a rock formation comprises: intake and discharge passages; a pumping chamber with a plunger therein; intake and discharge valve assemblies in the intake passage and the discharge passages. At least one valve assembly has a valve body movable into sealing contact with the valve seat by fluid pressure in the pumping chamber. The valve seat may have a cylindrical outer surface for mating reception in said intake passage, and a shoulder with a radially-projecting surface biased against a wall for sealing. A cushioning member may be interposed between the valve body and the valve seat. A seal may be interposed between said valve body and said valve seat, and configured so that between 35% and 60% of a sealing surface of said valve seat contacts the valve body.