rod 11 and this includes a seal ring 23 embodying the invention which is mounted in an annular groove 25 in the end cap extending concentrically around the opening 13 and having a bottom wall 27 ports 29 and 31 on opposite sides of the piston 9 illustrate means for admitting and exhausting pressure fluid and air as the piston 9 and rod 11 move in juxtaposition respectively to the cylinder wall 15 and the end cap well defining opening 13 referring to fig2 the piston seal 17 has flat parallel end faces 33 and 35 which are perpendicular to the seal axis an inner periphery 37 for static sealing to the bottom wall 21 of groove 19 and an outer periphery 39 that has an outwardly flared flexible rectangular lip 41 for dynamic sealing to the wall 15 of the cylinder body 3 the major portion 43 of the inner periphery 37 is a cylindrical wall concentric with the axis of the seal but the end of the periphery toward which the flared lip 41 extends has a stubby inwardly projecting triangular lip 45 that axially overlaps at its root end 47 the tip end 49 of the lip 41 in use the entire inner periphery 37 can serve as a sealing surface against the bottom wall 21 however the lip 45 forms a flare or interference which assures static sealing contact as well as forming a load point along the static periphery.