A seal assembly having a backup ring through which a piston reciprocates. There is a seal sleeve made of a non-abrading, extrusion resistant material that surrounds the piston at a location forwardly of the backup ring. The backup ring has an opening to receive the piston, and the opening is formed to define a radially and rearwardly expanding gas between the piston and backup ring to receive and discharge any seal material which may extrude into the gap. There is a positioning flange integral with the backup ring that surrounds the seal sleeve to press the seal sleeve into engagement with the piston, and there is an outer seal portion to form a seal between the positioning flange (or the outer portion of the backup ring) and the cylinder.