So, a little while back there was a thread on "do you clean your beer lines". Some, like me, responded "no". It's all sterile beer, I flush the line with "new" beer when I hook up a keg...why clean?
Well, today I installed 5 perlicks on the front, and when I bought them from AHS I also got some Beer Line Cleaner. I heated up some water, added BLC, and flushed 1L through each line. It ran clear almost instantly after I started it flowing...so I'm all "see? no buildup, these folks are nuts".
I let the lines sit for ~1h with the BLC in them still, then came back to flush another 1L through each. When I did this, I got BROWN for the first few hundred mLs....apparently I had enough build up in there that when they soaked, a lot more dissolved into the BLC.
SO, just because you think your lines are clean doesn't mean they are. Buildup DOES occur, even in an air-free environment. Flush your lines! I'm now a believer.