The theory of the gasket drying out due to 3 years of non-use is logical, but when I winterize, I use the RV anti-freeze instead of blowing out the lines. I fully run a gallon of the stuff through and let it sit in the lines.
Is not there a certain amount of lubricity of the RVAF that might offset the potential of the gasket drying out? Obviously, it is a water based liquid, but it feels slippery to the touch.
I am hoping that the gasket is somewhat still pliable and the re-tightening of it is a cure for a little more than just this summer.