Remove the valve. You'll see a big round cover with two holes in it. You need to fabricate something in order to fit perfectly in both holes. I used two old drill bits (bottom end), but I can't remember the size. Next, you need to weld some kind of rod or pipe to both of them. This is my special tool. The only thing left is to remove the cover (counterclockwise) and change that O-ring. If you can't match the O-ring put teflon around the thread of the round cover. This should stop your leak.
This is where 99% of them leak. The oil should be dripping right between the mounting bracket and the valve. Just double check that it isn't one of your pipes.