it do sound like there's no vent .. the vacuum developed prevents fuel fron flowing out.
When i got my bike it had been Kreemed .. ok, fine.. After a while i got around to replacing the gas cap's rubber gasket seal .. all of a sudden i have trouble. At that point i notice the cap itself was Kreemed. (The tank was venting through the old dried shrunken gasket.. but, no more)
soaked it in acetone to remove the kreem .. blew the two tiny vent holes out .. they seemed to be open .. but still a problem.
Finally i got some pliers and bend/broke the cap apart.. Every nook and cranny of that cap was packed with rust flakes so it couldn't vent..
when a tank's interior gets rusty you can be pretty sure the interior of the cap did too.