I just had the same problem with my 1990 300CE. What never got mentioned, even in the manuals, is that, for my car anyway, the reservoir for the rear brakes is at the right rear of the master reservoir and is not viewable from the fender so all you see is a full (front brakes) reservoir. I have never seen anything like this before in my 45 years of wrenching. Very frustrating for a while.
Cary Cooper