So this past weekend that ROAV's Wintergreen event my D1 started creating back-pressure and pushing fuel out of the filler cap. I know there are several things to start checking, my most immediate thought is the vent for the tank. Others have suggested the charcoal filter could be the issue. Anyone experience this before and have suggestions?

After I got down to 3/4 tank or less it seemed to just be pushing fumes out of the tank and not fuel. Once I got down to 1/2 tank +/- it didn't seem to be pushing out fumes?