This has been happening for some time now, and its starting to piss me off. On a full tank everything is fine, but once it gets down lower than a half, gas just will not stop comming up out of the vent tube. I have an extra long peice of it cut and looped and tied off, and gas will come out like it just out of the petcock damn near. I have to loosen the cap for it to stop and if I tighten it again, itll start again. If I dont tie it off its the same,. no matter what variable. Bad cap?