My stainless flask gets used every day, and washed nearly every day

I can't put tea into it, it always comes out tasting of plastic. Coffee is better, but I'd hesitate to offer it to anyone, it's an acquired taste.
As the only plastic part of the whole thing is the stopper/pourer it's hard to see what else to do but grin and bear it.
I sometimes make a jar of real coffee in an espresso pot and carry that in the flask and it tastes better simply (I'd guess) because the coffee is too strong to be messed with. However on those days I can go many hours without blinking.
GC