Green River RC,
When testing a tank with fuel in it you should pinch off the line from the muffler and blow real hard into the carb line. This will increase the pressure inside the tank and let you see if it is leaking. Of course when you quit blowing you will have enough fuel in your mouth to hydo lock a Hummer!

Tip...If you do this you will not forget to pinch the line, before you quit blowing, the second time you do it!