A boss assembly (28, 28a, 28b) for a pressure vessel, the boss assembly (28, 28a, 28b) including: a boss (30, 30a, 30b) including a bore with an inner surface and a first stop feature (36, 36a, 36b) adjacent the inner surface; a fitting (32, 32a, 32b) configured for at least partial insertion into the boss (30, 30a, 30b) to prevent fluid communication between an interior of the pressure vessel and an exterior of the pressure vessel, the fitting (32, 32a, 32b) including an outer surface and a second stop feature (38, 38a, 38b) adjacent the outer surface; a first seal positioned along the outer surface; and a second seal positioned along the outer surface; wherein the fitting (32, 32a, 32b) is movable between first and second positions relative to the boss (30, 30a, 30b), wherein: in the first position, the first seal is sealingly engaged between the inner and outer surfaces, the second seal is not sealingly engaged between the inner and outer surfaces, and there is no mutual contact between the first (36, 36a, 36b) and second (38, 38a, 38b) stop features; and characterized in that in the second position, the first seal is not sealingly engaged between the inner and outer surfaces, the second seal is sealingly engaged between the inner and outer surfaces, and the first stop feature (36, 36a, 36b) contacts the second stop feature (38, 38a, 38b). a boss (30, 30a, 30b) including a bore with an inner surface and a first stop feature (36, 36a, 36b) adjacent the inner surface; a fitting (32, 32a, 32b) configured for at least partial insertion into the boss (30, 30a, 30b) to prevent fluid communication between an interior of the pressure vessel and an exterior of the pressure vessel, the fitting (32, 32a, 32b) including an outer surface and a second stop feature (38, 38a, 38b) adjacent the outer surface; a first seal positioned along the outer surface; and a second seal positioned along the outer surface; wherein the fitting (32, 32a, 32b) is movable between first and second positions relative to the boss (30, 30a, 30b), wherein: in the first position, the first seal is sealingly engaged between the inner and outer surfaces, the second seal is not sealingly engaged between the inner and outer surfaces, and there is no mutual contact between the first (36, 36a, 36b) and second (38, 38a, 38b) stop features; and characterized in that in the second position, the first seal is not sealingly engaged between the inner and outer surfaces, the second seal is sealingly engaged between the inner and outer surfaces, and the first stop feature (36, 36a, 36b) contacts the second stop feature (38, 38a, 38b). in the first position, the first seal is sealingly engaged between the inner and outer surfaces, the second seal is not sealingly engaged between the inner and outer surfaces, and there is no mutual contact between the first (36, 36a, 36b) and second (38, 38a, 38b) stop features; and characterized in that in the second position, the first seal is not sealingly engaged between the inner and outer surfaces, the second seal is sealingly engaged between the inner and outer surfaces, and the first stop feature (36, 36a, 36b) contacts the second stop feature (38, 38a, 38b).