Originally Posted by Jeremy5848

Your vacuum leak is probably somewhere in the engine compartment. (With a W124, the door locks are on a separate vacuum pump (under the rear seat) so at least you don't have those to worry about.)

Go through the engine bay, tracing the big vacuum line from the pump on the left front of the engine to the brake booster. Check every line that tees off of the big one and every place they go. The vacuum lines themselves are hard plastic and fairly rugged, although they can be burned or cut. Replace any lines that look damaged.

The rubber connectors that connect several lines can get leaky over the decades and should be replaced if they feel soft. ("Soft" is a relative term and you really need a new one for comparison. They are a few dollars apiece -- you can get them from Phil.)

While you're looking over the vacuum lines, remember that the system is full of restrictors -- little things that look like plastic pieces joining two lengths of tubing. Not only does each one need to be there, but there are several different "sizes" and each one needs to be correct for the system to function correctly. A diagram of all of the vacuum lines is a valuable asset. They are usually included with the factory service manual and may be on line some place.

You should also check the "ignition" switch as it has a vacuum line -- that's how the signal gets to the engine to shut it off. Make sure the line is in good condition and isn't full of oil -- that can happen if your vacuum pump is failing. Trace the line from the switch to the IP, where there is a vacuum actuator that closes the IP, that's how the engine is shut off.