Tygon seems to harden when it's been in gas then dried out, I try to leave just a little fuel in my planes and I've had the same piece for 2 years in one of my planes and it still clunks around fine. By just a little bit I mean I have a Sullivan tank and I just leave gas in the little front part as I store the plane on it's nose (any more and it seeps through the Dubro stopper or gets through the vent line) since I store them in a tiny bedroom that's also my office