A valve assembly (200;400;600) for a fuel injector comprising: a) a sleeve member (220;420;620) having an interior chamber (222;422;622); b) a piston member (224;424;624) supported for movement within the interior chamber (222;422;622) of the sleeve member (220;420;620) and delimiting an upstream pressure side of the valve assembly and a downstream pressure side of the valve assembly; c) a dynamic sealing device (230;430;630) mounted to one of the sleeve member (220;420;620) and the piston member (224;424;624) for sealingly isolating the upstream pressure side of the valve assembly from the downstream pressure side of the valve assembly, the dynamic sealing assembly including: i) an annular seal member (232;432;632) having a generally C-shaped cross-section including opposed upstream and downstream legs (233,235;433,435;633,635) defining an internal cavity (234;434;634) with an opening facing toward the upstream pressure side of the valve assembly, wherein the upstream leg (233;433;633) has a radial length that is greater than a radial length of the downstream leg (235;435;635); and ii) a spring member (238;438;638) disposed within the internal cavity (234;434;634) of the seal member (232;432;632) for providing a constant sealing force to the seal member; characterized in that; the upstream leg (233;433;633) of the seal member (232;432;632) includes a retaining foot (237, 437) that extends angularly away from the upstream leg (233;433) in an upstream direction and wherein an axial collar (250;653) or a threaded retainer secures the sealing device (230;430;630) in a seat (240;440;640) formed in a head portion (226;426;626) of the piston member (224;424;624). a) a sleeve member (220;420;620) having an interior chamber (222;422;622); b) a piston member (224;424;624) supported for movement within the interior chamber (222;422;622) of the sleeve member (220;420;620) and delimiting an upstream pressure side of the valve assembly and a downstream pressure side of the valve assembly; c) a dynamic sealing device (230;430;630) mounted to one of the sleeve member (220;420;620) and the piston member (224;424;624) for sealingly isolating the upstream pressure side of the valve assembly from the downstream pressure side of the valve assembly, the dynamic sealing assembly including: i) an annular seal member (232;432;632) having a generally C-shaped cross-section including opposed upstream and downstream legs (233,235;433,435;633,635) defining an internal cavity (234;434;634) with an opening facing toward the upstream pressure side of the valve assembly, wherein the upstream leg (233;433;633) has a radial length that is greater than a radial length of the downstream leg (235;435;635); and ii) a spring member (238;438;638) disposed within the internal cavity (234;434;634) of the seal member (232;432;632) for providing a constant sealing force to the seal member; characterized in that; the upstream leg (233;433;633) of the seal member (232;432;632) includes a retaining foot (237, 437) that extends angularly away from the upstream leg (233;433) in an upstream direction and i) an annular seal member (232;432;632) having a generally C-shaped cross-section including opposed upstream and downstream legs (233,235;433,435;633,635) defining an internal cavity (234;434;634) with an opening facing toward the upstream pressure side of the valve assembly, wherein the upstream leg (233;433;633) has a radial length that is greater than a radial length of the downstream leg (235;435;635); and ii) a spring member (238;438;638) disposed within the internal cavity (234;434;634) of the seal member (232;432;632) for providing a constant sealing force to the seal member; characterized in that; the upstream leg (233;433;633) of the seal member (232;432;632) includes a retaining foot (237, 437) that extends angularly away from the upstream leg (233;433) in an upstream direction and