There should be a crush washer, or delrin washer where your c02 regulator connects to your tank. I one time hooked my my regulator up without it and got a leak roughly the speed of the one you described. Other then that the above suggestions should go a long way to helping you find the leak. I'd use the soapy water method over submerging your whole C02 tank, and I don't have any technical reason for saying that , but I am under the impression that tilting a C02 tank on it's side while in use is not a good thing. (again, I have absolutely no technical proof, this is merely what I've come to believe over the years-maybe someone else can give you technical information as to why I'm right or wrong about this?)