If you here a wheezing sound from behind the steering wheel and your engine is slow to turn off you could have a leak or defective vacuum shut off valve located on the ignition switch (located right behind the steering wheel under the dash). To check this locate the two brown vacuum lines under the hood. One of these lines goes to the shut off valve located at the rear of the injection pump. The other brown line goes to the shut off valve located on the ignition switch. Disconnect the brown line going to the ignition switch and with the engine running apply vacuum to this line using your Mityvac. The engine should off immediately. If it does not, the next test is to apply vacuum to the brown line going to the shut off valve on the injection pump while the engine is running. If the engine shuts off immediately and you are able to hold a vacuum for a period of time (at least 15 minutes) then your IP shut off valve is OK, and the shut off valve on the ignition switch is suspect. This valve can be accessed by removing the instrument cluster and is easy to replace and cheap.

As you have realized most functions on the W123 diesel cars are controlled by vacuum and vacuum leaks can occur in many different places. Another common source for vacuum leaks is the vacuum reservoir at the tube leading into the reservoir. Test the reservoir with your Mityvac to see if it holds vacuum. It will take several pumps of your Mityvac to create a vacuum in the reservoir.

Another check is to check all of the underhood items controlled by vacuum. From the main vacuum line going to the brake booster from the vacuum pump there are two vacuum nipples. Remove the hose on the nipple closest to the front of the car and apply vacuum to the hose. Vacuum should hold for at least overnight. If not, you will need to isolate and check each underhood item individually (EGR valve, transmission modulator, etc.).

Some good shops use a smoke machine to detect leaks in the vacuum lines (pressurized smoke to pinpoint a leak) or they pressurize the vacuum likes with air and listen for the leak.