A seal assembly is disposed in an annular recess in a cylinder chamber between a pump cavity and a pump exterior. The seal assembly includes a main seal and a back-up seal and seals to a pump plunger disposed within the cylinder chamber. The back-up seal includes an extrusion recess extending from a surface of the back-up seal adjacent the main seal along an inner or an outer diameter of the back-up seal. During pumping operations the plunger strokes through the cylinder chamber causing the main seal to extrude into the extrusion recess to form a seal between the plunger and the cylinder chamber.