Maybe check the vent line and make sure there are no restrictions. On the "Tanks Inc" site, they say not to use the commonly done curly cue vent that some people make as it can collect liquid in the low spots, from the fuel condensing after cooling off, causing a restriction in the venting. My neighbor had a tank made out of aluminum, bad choice btw, with the vent done that way. He has fun filling it sometimes because that vent is restricted. I need to make a straight one for him down the road.