Abstract: In a subsurface positive displacement pump used in the petroleum industry for pumping crudes characterized by a high degree of suspended solid particles, a ball check valve comprising a valve ball and a valve seat assembly wherein the valve seat assembly comprises a hollow cylindrical body defining an orifice through which pumped fluid passes, the hollow cylindrical body being provided with a recess on the surface facing the valve ball in the region defining the orifice wherein an insert which forms the actual valve seat for the ball valve is secured in the recess. The insert is formed of a material having a hardness greater than the hardness of the material from which the valve ball is formed which material in turn is characterized by a hardness greater than that of the hollow cylindrical body.