O-ring/back-up ring systems that are suitable for use in high pressure applications, including those of 70 MPa automotive hydrogen storage applications, to form a seal between first and second connector bodies. Such sealing systems comprise a back-up ring having a base portion and a flange portion, wherein such ring is configured to prevent extrusion of an O-ring into a gap existing between the back-up ring and a groove into which the back-up ring and O-ring are disposed. High pressure gas storage systems comprising such sealing systems, and methods of sealing a connection in a high pressure hydrogen storage system using such sealing systems.