If you look I think you'll se that the vent tube just goes to the filler neck... and is not vented to the out side (emission stuff)... You may have put to much fuel in the tank and as it heats up during the days sun the fuel well expand and then over fill the tank pushing it out the cap.. with closed systems anymore you need to just not put so much in the tank...
just my .o2
Tim

______________________________________
64 International 4dr. truck w/466