I have a four tap kegerator with perlick 630ss. I have a cider on one of the taps and noticed it had foamy pours. I have 10' of beer line so that's not the issue. I tested a pour while having the door open and noticed that when I close the tap the liquid in the line starts falling back towards the keg which means there's air in the beer line. I took the tap apart and relubed but it's still doing the same thing. I'm going to replace the line but can't figure out how the beer is going back into the keg when there are no visible leaks in the tap or keg. Any suggestions?