I had the same problem with my 83 300D. I had to remove the line at each operator with a electric vacum pump supplying a constant vacum to locate the faulty operator. It turned out o be the trunk lid lock operator. you need a vacum pump that will shut off when it reaches full vacum. It was time consuming. I used a vacum pump off of a ford f800 with a cummins diesel, it works off of twelve volts and has a internal switch that shuts it off at a preset vacum level. It also has a internal check valve.Kinda crude but it worked well and was cheap, I had one laying in my tool box. Just made the jumper wires. then I tied into the vacum line for the central locks under the hood.You need to block the vacum lines of individually until the system will hold vacum. There may be a better way but I can't see one.Just my expieriance.
auto\hydraulic mechanic 20 yrs