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How common is this problem on 20 year old coaches, are people changing these, or carrying a rebuild kit.

Probably depends on if you have the plastic exhaust or metal. I would say if you have plastic exhaust ports go ahead and pre-emptive replace them. Mine would be 28 years old, and my coach is a southern coach and has little to no corrosion etc under it but it was in a dry environment, so maybe that made them fail earlier? What I do know is that it will drain your air system quickly and put you on the side of the road. You still have air brakes thankfully but you will lose air in your airbags. On mine that is how I noticed, by the smell and sight of the rubber rubbing from one of my rear tires rubbing on the frame of the bus. The air light and buzzer did not come on, I need to investigate that a bit further.