A sealing system for an outlet of a plastic-lined cylinder has a plastic liner outlet extending into a bore of a boss. An insert is engageable with the bore, forming a primary seal between the insert and portion of the liner outlet. Further, a tapered compression surface of the insert engages a tapered bore portion of the liner outlet adjacent the primary seal for compressing the liner outlet and closing any annular assembly clearance at the primary seal for obviating opportunity for seal extrusion under pressure. An axial position in the boss, and dimensional integrity, of the liner outlet is maintained by engaging an annular distal end of the liner outlet in the bore by a stop. The stop can be integral with the boss. Reverse extrusion of the primary seal due to trapped gas and rapid decompression of the cylinder can be defeated by providing a leak path.