Originally Posted by waterfront
...the vent line had a lloop in it that was causing the problem. ...
That loop in the vent line is there to keep sea water from being forced up the vent line and into your tank. Unless it's kinked, it should not impede tank venting.
I have one tank that seems more temperamental than the other when it comes to venting. After changing lines and vents, I finally settled on installing vent whistles that sound while venting, but go silent as soon as fuel enters the vent line. I also keep the filler neck of the gas pump spout out an inch or so to allow venting from the fill tube. When the whistle stops, I stop pumping. Works well for me.