Abstract: A pump piston construction for mud pumps and the like incorporating a resilient sealing member that establishes a pressure enhanced seal between the piston and pump cylinder. A bearing and anti-extrusion member composed of a relatively hard and flexible friction resistant plastic material is structurally interrelated with and may also be bonded to the sealing member. The anti-extrusion member is supported by an annular rigid flange of a piston hub about which both the sealing member and anti-extrusion member are received. The anti-extrusion member serves as a bearing to minimize wear of the piston and cylinder and is yielded radially outwardly responsive to fluid pressure transmitted through the resilient sealing member and functions to prevent extrusion of any of the resilient material of the sealing member into the space between the piston and cylinder.