Hello, my transmission on my 190E leaks on the passenger side where a big round metal seal is located, toward the front. This seal will move in and out and is attached with a metal locking ring. What is supposed to keep fluid from leaking out, an o-ring, a gasket, or what? At first I thought it was the pan gasket but it wasn't. I do all my own work but the transmission is foreign to me. Please help. Thanks.