Abstract: The back-up rings of the present invention are seated on opposite sides of a sealing ring within an annular groove of a hydraulic piston. The sealing ring is in frictional engagement with the interior surface of the hydraulic cylinder, and the back-up rings are positioned within the cylinder groove and on opposite sides of the sealing ring. At least one of the back-up rings includes a vent hole extending axially therethrough to permit the alleviation of any fluid pressure accumulating between the two spaced apart back-up rings. As an alternative to the axial vent hole, a groove may be provided on the interior perimeter of the back-up ring or the back-up ring may include a complete break therein for permitting the venting of fluid trapped between the two back-up rings. The method of the present invention includes venting any fluid which is trapped between the two back-up rings during reciprocation of the piston within the cylinder.