There's a lot of post on this but no answers after the person finds the problem. Hate to beat a dead dog but is loosening the cap enough for vent or is there an internal component with the tank that would cause failure
If it is a metal tank there are 2 small pins.-----------When you connect the gas line you push in those 2 pins.---------They are valves and one lets the fuel out and the other lets air in.---------------Make sure the o-rings on those valves are good.-------If one pin sticks out further than the other the tank needs attention.