After checking what Mode One suggested, also confirm that when your brother assembled the tank he didn't cut a hole in the pickup line. You shouldn't be getting a constant stream of bubbles in the fuel line and one should NEVER FLY with this condition. Out of curiosity, what brand and size of fuel line is being used from the tank to the carb? If it's not getting a tight fit at the tank end, the suction could pull in some air. Also confirm that the lines are hooked up correctly. If you can't get it working right in the plane, remove the tank and engine and try the same setup on a test stand.