Fuel tank vent.

If you hooked up one end of the tube to the sender extra port, and to the filler neck port you do not have a vent. Either you have a vented gas cap or there is another opening in the tank somewhere.
Yep, plugged the high out on the sender, used the tank bung to plumb outta. It was on/ near the top of the tank.. I'd go get a pic, for reference, but I just got outta the shower. Now I Remember though, the Tank had the Lil nipple that had to be dealt with.