A seal system for a pressure vessel which includes a substantially rigid outer shell with at least one opening therein, an inner liner disposed within the outer shell against the inside surface thereof, an opening aligned with the opening of the outer shell and a boss disposed in the opening of the outer shell. The boss includes a neck portion for fitting in the opening of the outer shell, a flange portion extending outwardly from one end of the neck portion and having an upper surface and a lower surface, and a generally cylindrical hollow substantially aligned with the outer shell opening and having an inwardly projecting annular shoulder. The lip segment of the liner extends radially inwardly under the lower surface of the flange portion of the boss and then upwardly into the hollow of the boss, along the interior wall of the hollow until a portion of the lip segment rests on the shoulder.The seal system includes an attachment or coupling device having a generally cylindrical, downwardly extending lower insert section. The lower insert section includes an annular bottom wall in which is formed an annular recess opened downwardly. An upper head section is formed at the upper end of the insert section and includes a radially extending flange. The insert section is dimensioned to fit in and be received by the hollow of the boss above the shoulder. The seal system also includes a resilient O-ring for disposition in the recess of the insert section of the coupling device to protrude from the recess a sufficient distance to contact and seal against the lip section of the liner resting on the shoulder of the boss hollow, when the insert section is fitted into the hollow.