Re the monovalve - I had the same problem with the MTC valve. No heat over about 10 mph. I think that because the diaphragm did not fold up neat like the original, the increased water flow at more than an idle sucked the valve shut. I got a genuine Mercedes one for about $70 from my almost local Mercedes dealer, and it works perfectly. Could see a huge difference in how the diaphragm material tucked away when the valve was in an open position. My impression was that probably all of the aftermarket ones are made by whoever makes the MTC.

Replacing the heater valve body won't do any good without a good valve. I would certainly try the original valve before trying to redesign the whole system.

RE black hose - If the car is a W123 (I'm not sure ab out an SD) and if it's a half inch or so thick, not a vacuum line, it should be attached to the plastic line from the sensor under the rear view mirror on the top of the dash. There was a foam pipe about 4 - 6 inches long connecting the two plastic pipes that may have rotted away. I used foam water pipe insulation on mine, and my climate control started working as it should.

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1985 300TD, 367,000 daily driver. EGR, ARV, trap deleted