Carbon is what jams them up. Once you break the carbon loose with Kroil and several taps with a hammer, it should become loose enough to function properly for another 50 years.
Taking it apart and replacing the shaft is not the easiest of undertakings. To replace the shaft, you will need to do a lot of stud and bolt breaking to get to the flapper. Then you will need to separate the flapper from the shaft. Then you will need to weld the steel flapper back on to the stainless steel shaft. This will require a TIG welder for an easy to do good weld.
I would reserve the replacement of the shaft for when there is a lot of slop between the manifold and the shaft, or for when the slotted end is broken, preventing you from putting the spring in place.
Carl
