Air leaks can be a real bugger to track down and fix. That is true on any air operated coach. The pressure regulator you are talking about is somewhat different than the ones I have seen on Newells in the age range of yours. I wonder if it has been replaced. I have heard of a few cases where someone had a leaking regulator and put a screw into the orifice to stop the venting. That would result in an inability to reduce the pressure once it was increased.
