A boss assembly for a pressure vessel includes a boss, a fitting, a first seal and a second seal. The boss includes a bore with an inner surface and a first stop feature. The fitting is configured for at least partial insertion into the boss to prevent fluid communication between an interior and an exterior of the pressure vessel, the fitting including an outer surface and a second stop feature. Each of the first and second seals is positioned along the outer surface. The fitting is movable between first and second positions. In the first position, the first seal is sealingly engaged between the inner and outer surfaces and the second seal is not sealingly engaged therebetween. In the second position, the first seal is not sealingly engaged between the inner and outer surfaces, the second seal is sealingly engaged, and the first and second stop features mutually contact.