I wish that were the problem, but as with my first Valk you can actualy see the gas in the keyhole when you slosh it around.
Not being critical here but that would make me think you're filling the tank up to where the overflow ought to be draining so I'd suggest check the overflow.
Also, with a full tank of gas sitting in the sun, the gas will expand and could cause the same affect.
I'd suggest not filling the tank so completely will correct the problem.
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