well, my opinion...i don't know how much you're getting but even as a young guy i had always heard to never store or leave a gas tank half empty or less as moisture collects , especially during winter months. i also adhere to the belief to always topping off my gas tanks to further help eliminate condensation.
I always thought gas caps were just vented, period, at least vintage vehicles. Newer cars appear to have " one way" caps but that's because of the closed fuel systems in association with the pcv systems which vent over pressure. I've gotten the rush of air with some cars when removing the cap which usually means something in the pcv system is malfunctioning.
Plus, when burning fuel, that creates negative pressure which pulls in outside air containing moisture, which would be the biggest source. So, sitting static, should not matter, except for normal condensation.
I flew aircraft for a living and when flying the smaller piston aircraft we always had to drain the tanks as a preflight item because of condensation......you'd be surprised at the water that could collect.....
probably not helping, but, just how much water are you getting?
