what was getting in there? I've never found fermentation to leave any residue on surfaces provided the krausen didn't come into contact with the item in question.
I never found anything in the assembly but my beers had this lacto/acto cidery wang infection that I could not track down.
My local microbrewery also confirmed that you can get an infection from a dirty blow off tube, without the beer touching it, which I thought was crazy... but hey he should know more than me.
On their big conicals they clean the blow off tube assembly/pipe with chemicals and sanitize it before each brew.
I have also have gotten infections from a dirty CO2 line in my kegerator-frige, all be it my fault that some beer backed up in the line, but now I anal about bleeding off kegs before I put on the gas to keep the gas from the kegs from backing up in the lines, etc.(hope that made sense...)
I have also read that others have had this issue and some guys, like me replace all CO2 lines every 6 months just to be sure to eliminate this point of infections. ( along with sanitizing the connectors/manifold/etc)
Again
Your Malt May Vary... have fun with the valves.
Kevin