I have an '86 420 SEL, 200K miles.
Started fine, and kept stalling when I put it into gear. So I started it, and took a look around. There is an air leak sound coming from behind the rear passenger tire (near the fuel filter). Sounds like I have a hole in the tire - best way to describe it, but no it is not the tire. Couldn't find the leak though.
After a few minutes of looking around, I put it into gear and it ran fine for over an hour drive. I parked it, left it overnight, and can't get it started.
I just got the car a month ago - know very little about Mercedes fuel and vacuum system, but I don't want to tow it to a mechanic if there is just a hose somewhere that dry-rotted.
Anybody have any ideas of what this might be?
The AC went out too just recently - don't know if this has anything to do with it, but I figured it it might if there is a vacuum problem.

 

Don't have any experience on MB's this old. However I think you have found the problem UNLESS the noise you hear is just the electric fuel pump running.
It appears that the line used to connect the tank to the underbonnet charcoal can may be leaking. Assuming that your car has a charcoal can for fume purge.
Such a leak is likely to give symptoms like those you describe.
Need to get the car on a ramp and trace all lines from tank to engine bay. The problem one will be the line which is NOT carrying fuel to the engine but you should be able to determine this by following the noise.
Good luck.
Stuart