Regarding your original question, I see we are both still chasing that elusive DIY for monovalve replacement. I haven't found it either. Maybe after this weekend I can help. I bought a monovalve from FastLane and now know what one looks like. My son and I will attempt a swap this weekend and try to take and post a few pictures.

On my 1988 260E (which apparently is really the same as a 300E) the monovalve is located under the hood on the passenger side within inches of the battery. It's a small inverted canister over two inches high and over an inch across (from memory but not measured). It has two hoses connected to it, and an electrical connection on top. The whole unit seems to be held in place by two rubber pointy things (I think that's the right technical term). I haven't figured out how to remove or unsnap those yet. I'm sure it's very simple once you know how to do it. Also, I don't know if I have to clamp off the hoses to reduce massive fluid loss of if I can just disconnect them. We'll see. If anyone has input...what about the two rubber pointy things and the hoses?

Thanks,

Techie9