I have read posts before where there is no vent to the tank and the tank collapses, it literally shrinks. This could be happening to your tank, although the lid should take care of any pressure over the threshold. But seriously if you can't diagnose yourself get it to the dealer and have it checked, I would start with the purge valve, seems like it may not be closing and allowing constant vacuum to the tank for vapor creating a negative pressure in the tank. Good luck!
B

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Brian W. Heitman

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