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Yup, this is what air leaks do! They come and go, one day its find next day it drives you nuts. Agree with Jim, replace it now and hopefully that will take care of it. 

I am pretty sure the air dryer on my coach was never serviced, it took me several hours to get the filter off the dryer. I ended up getting a large chain wrench to get it off. There was all kinds of crud in it, I had to take it apart and clean it all out. I went ahead and put a new purge valve on it too while I had it apart, they are a common issue. I carry two spare purge valves in my spares.