My system is much better this year (year 4 since new) than it was last year. I’m thinking one of my treatments early last year (leaving chlorine mix in too long, or vinegar) damaged the inside of the PVC flex lines making them continue to release afterwards offending molecules of some kind, and enough time has now passed that the inside of the lines now has a chemically stable layer again. It still tastes/smells a little plasticky when I taste a room temperature cup from a tank loaded a few days earlier, but that’s a huge improvement over last year when anything I washed, including myself, would stink of plastic afterwards.
I will go easy on the chlorine and not use vinegar again.