DO NOT fill the tank Completely, until you find the leak. Just pump a gallon or 2. Then use soapy water around ALL the fittings on the tank. The tank should have more than enough clearance to cawl under it to soak the bottom fittings. That bottom fitting is usually the tank valve relief. It SHOULD reset after it trips, but it may now be faulty. I would replace it if that is where the soapy water bubbles. If you only put a gallon or so in the tank, then bleeding it off by emptying the tank will not take long. The tank has to be empty of liquid and vapor to replace that valve. I would highly advise you to have a LP company replace the valve after you verify the problem. Doug