[xcntrk]

Do you think it might be a venting or vacuum while pumping problem?

Not sure if your tanks and caps are the same as mine; but I have a problem with both my tanks building pressure as the temp changes. There doesn't seem to be a dedicated tank vent and the gas cap is used to allow vacuum while draining and to relieve pressure, however you have to have the caps in the right position to enable the self-venting.

Try this simple test; Crank the gas cap down tight to where it clicks. At this point you have entirely sealed the tank and it has no vent (from what I can tell). Now, twist the cap counter clock-wise as if to remove it, but stop once you feel pressure from the engagement mechanism. You'll notice the cap will twist maybe a turn and a half before it starts to unscrew. This seems to allow the cap to operate as a vent for the tank.

Whenever I fill my tanks, I perform this procedure and no more venting issues. They must be self-venting caps and screwing them on all the way tight without relieving the venting mechanism prevents them from operating correctly. Good luck!