This is for a 1981 240D. I believe that I have traced the vacuum leak to a green/yellow tube that connects just above the yellow check valve that goes into the passenger compartment. I took off the bottom panel to the dash on the driver side and saw that it went up, so I took the insterment cluster out and saw that it continued up into the insulation at the top of the dash. My guess is that it controls a ventelation valve, probably the defrost valve.

Does this sound right? And assuming it does, how do I get to valve to replace it.

Is any additional information required?

The rest of the system seemed to check out.