I have a 1985 300D that has sat for about 6 months. I have changed all the filters and have it running, but it will not stop when I turn the switch off. I understand the problem could be a leak in the vacuum line, the vacuum pump, or the vacuum switch behind the dash. Can anybody help me with trouble-shooting procedures? Which vacuum line should I be looking at, etc.

 

Search the forum. You'll find the answer.

 

There is a hand Shutoff Lever up near the Vave Cover that you can push to shut it off.

There is a Vacuum Shutoff Valve on the Fuel Injection Pump, A Vacuum Shutoff Valve mounted on the Steering Wheel Lock and is actuated by the Key when you Turn it.

Leaks in the Vacuum Locking System or disconnected Hoses under the hood are most often the cause of the Vacuum Shutoff not working.
If when you try to brake the Pedal effort is very hard then I would suspect the Vacuum Pump or the Main Vacuum Line.