I only had one AD-9 purge valve failed on very old truck - replaced it and it was good to go again, had several AD-SP with brocken snap ring groove that holds purge valve, AD-IS had only couple, constantly leaking and very expensive on parts. One time needed only governor for that one and dealer was hell bent on selling me the whole thing for more then $500, claimed that you cannot put regular $40 governor on that one. Frankly I didn’t investigate weather you could or not, because it was something like third time I had problems withi it - threw it away and plumbed $145 AD-9 in place of it moved governor to the compressor where it belonged! Yes it took more work and buying some air lines and fittings, but that one was a long haul truck, and I was not about loosing money on tow changes and being late because of something stupid like this. Same thing with replacing all push in ( especially plastic ones ) connectors with brass threaded ones, if it leaks it goes straight to garbage can, and threaded goes in place of it!
And another thing with these - it pissed me off when they give you problems and seems to pick the worst weather possible, so you have to crawl under the truck in mud and wet snow!