[spdbrake]

Typically the fuel caps are vented and it sounds like your is not.

Another issue if the tank vent line (that usually runs from the top corner of the tank to the side of the fill line) has a low spot (right after it leaves the tank, typically look will like a J vs the required L shape) it will cause the fuel to start backing up the fill pipe as temperature increases. If can be exasperated even more if the return fuel line dumps into the fill line as well.