NOVELTY - The system has a flange portion (105) located on an inner surface (104) i.e. inner circumference, and a proximate surface (102) of a base portion (101) of a back-up ring (100). The back-up ring engages an O-ring and cooperatively forms a seal between connector bodies e.g. cylinder surface and housing inner surface. The back-up ring prevents an extrusion of the O-ring into a gap between an outer surface of one of the bodies and an inner surface of the other body and into another gap between an inner surface of the back-up ring and the groove inner surface when exposed to a positive pressure.    USE - High pressure gas storage system e.g. 35 megapascal or 70 megapascal hydrogen storage system, for a fuel cell vehicle (all claimed) and a hybrid vehicle. Can also be used for medical gases, industrial gases, natural gas and scuba tanks.    ADVANTAGE - The back-up ring prevents the extrusion of the O-ring into the gap between the outer surface of one connector body and the inner surface another connector body and into another gap between the inner surface of the back-up ring and the groove inner surface when exposed to the positive pressure such that the connector bodies can be sealed to each other, thus preventing leakage of the high pressure gas fuels.    DETAILED DESCRIPTION - The connector bodies are portions of a pressure regulator, an excess flow limiter, a shut-off valve, a check valve, a flow valve, or a pipe fitting. The O-ring is made of a material that is selected from nitriles such as acrylonitrile-butadiene, hydrogenated nitrile, and carboylated nitrile, rubbers such as ethylene propylene diene monomer, ethylene propylene rubber, chloroprene rubber, butyl rubber, and silicone rubber, fluorocarbon, fluorosilicone, perfluoroelastomer, tetrafluoroethylene-propylene, ethylene acrylate, polyacrylate and thermoplastic polyurethane. The base portion is formed of a material that is selected from nitriles, poly tetra fluoro ethylene, poly ether ether ketone, ultra-high molecular weight polyethylene, polyurethane and fluorocarbon. The flange portion is formed of a material that is selected from nitriles, acrylonitrile-butadiene, fluorocarbon, thermoplastic polyurethane, rubber, ethylene propylene rubber, ethylene propylene diene monomer, chloroprene rubber, butyl rubber and silicone rubber. INDEPENDENT CLAIMS are also included for the following:    (1) a back-up ring    (2) a method for sealing a connection in a 70 megapascal hydrogen storage system.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a back-up ring with a base portion and flange portions.    Back-up ring (100)    Base portion (101)    Proximate surface of base portion (102)    Inner surface of base portion (104)    Flange portion (105)