Hard rubber molded contoured Back-Ups are routinely produced in a 90 durometer Nitrile and Viton. This provides sufficient elasticity to permit stretching over the major diameter of a piston and then snapping back into the gland groove cut into the piston. Back-up Rings are used in conjunction with O-rings and seals for both static and dynamic sealing applications. They prevent extrusion of the O-ring or seal when it is subjected to high pressure, and/or larger extrusion gaps.