its unlikely air is getting in the system. Nitrous under pressure will be in liquid form....but there will always be a certain amount that will "boil off" and stay gas. Wether its in the lines or in the bottle itself. Thats why you have a purge in the first place...and as you see the nitrous guys at the track....purge on every run. Gas will build up in the lines...but its ok...its nitrous oxide gas.