A high pressure piston for use within a liner for a reciprocating pump comprises an elastomeric seal and an annular bearing ring. During the pump's pressure stroke at least a portion of the bearing ring elastically expands radially to narrow or close the extrusion gap, thus tending to block elastomer extrusion and establish sliding contact between the bearing ring's outer surface and the liner. An extrusion control groove located radially inward from the peripheral bearing ring contact surface of certain embodiments also tends to block elastomer extrusion. Under reduced pressure on the pump's return stroke, the bearing ring elastically contracts radially, slightly widening the extrusion gap. On the pump's return stroke the slightly wider extrusion gap allows cooling water directed generally toward the proximal flange of the piston hub to better cleanse, as well as cool, the liner wall.