You probably have blockage in the vent line. In the winter the gas tank won't expand of the fumes cause its cold, in the summer tho the fumes won't be able to escape, just like a jerry can on a hot summer day without opening the vent cap.

 

thats what i figured, after trying, and doing, every thing else. i have the vent lines that run to the charcoal canister and checked the line at it, sure seemed clear to me. i do know there is a plastic valve on top of each tank. i assumed they connected together and went to the charcoal canister. i can put a vacuum to the line and it free. if its the valves on top of each gas tank, well cant figure how both would be bad. guess i need to do some more checking.