We changed the fuel tank design around 640 so that the filler neck on the tank was on the side of the tank, not the top. The vent was moved to the back of the tank at that time.
The slope of the vent line is small toward the tank end, and therefore fairly sensitive to getting blocked. If you inadvertantly get fuel into the vent line, it might take a few seconds for it to drain back down into the tank. Just be carefull to avoid getting fuel into the vent in the first place.
