is installed such that the seal assembly 10 contacts cylinder wall 38 sealing ring 11 deforms to the shape illustrated in fig3 sealing ring 11 has been deformed such that its surface 24 bulges into triangular space 36 triangular space 36 shrinks but maintains a triangular shape side face 18 of the sealing ring bulges into space 40 when a pressure of approximately 500 psi is applied to piston 14 sealing ring 11 deforms to the shape shown in fig4 sealing ring 11 bulges into spaces 36 and 40 this deformation of sealing ring 11 urges back up ring 12 radially outwardly into sealing engagement with cylinder wall 38 triangular space 36 has disappeared while space 40 on the pressure side of the assembly has been substantially enlarged space 42 is now very small when low pressure is restored the seal assembly returns to the configuration shown in fig3 the sealing ring should be composed of a significantly softer material than the back up ring the workable range of hardness is from about 60 to 90 durometer the preferred material for sealing ring 11 is a resilient rubber the back up ring 12 should be composed of a low friction material which is not rock hard but rather slightly resilient virgin or loaded polytrifluoroethylene sold commercially as teflon is especially well.