There are occasions where it is good practice to retain older things...sometimes, it is just impossible or impractical. I think you may have reached the impractical and maybe the impossible stage - it's probably time for a new valve. While sometimes, you can get parts long after a thing has stopped being manufacturered, but that depends on how many were built, whether the company is still in business, and whether it is of good enough quality to justify fixing rather than throwing it away.
Some of the stuff was junk when it was built. Some designs are much easier to repair than others, and some are not really designed with maintenance or repair in mind.