The reason they expire is nothing to do with seals, in fact generally there are no rubber seals at all exposed to CO2 in fire extinguishers it is either metal to metal valve or sometimes Teflon to metal. It is to do with pressure testing. Most pressure tanks will have a repeat pressure test date usually 5 to 10 years after date of manufacture. For a fire extinguisher cylinder it is 10 years, thus to use a fire extinguisher as an extinguisher after 10 years, it must be emptied and the cylinder pressure tested. Generally it is not worth it and the extinguishers are scrapped or handed to fish keepers....