A valve assembly for a fuel injector that includes a sleeve member having an interior chamber, a piston member supported for movement within the interior chamber of the sleeve member and separating an upstream pressure side of the valve assembly from a downstream pressure side of the valve assembly, and a dynamic sealing device mounted to one of the sleeve member and the piston member for sealingly isolating the upstream pressure side of the valve assembly from the downstream pressure side of the valve assembly. The dynamic sealing assembly includes a PTFE seal member having a generally C-shaped cross-section defining an internal cavity having an opening about the outer periphery thereof the seal member, and a helical spring disposed within the internal cavity of the seal member for providing a constant sealing force to the seal member. The dynamic sealing device is positioned in the valve assembly so that the opening of the internal cavity of the seal member faces away from the upstream pressure side of the valve assembly and towards the downstream pressure side of the valve assembly.