I found the blue/brown vacuum line and the fuel shutoff valve, which is below the other gizmo that i thought it was. In front of the oil FILTER, not cooler housing. When i applied vacuum, it smartly shut off at about 5 or 6 on the vacuum pump gauge. There is only about 3 coming off that blue brown tube, when i turn the key off. So i will follow the other excellent suggestions here to track down where that vacuum is getting lost, and report back later.

1. Congratulations on achieving this first step. You're on your way.
2. With all due respect to previous posters to this thread, the above results are PRECISELY why it is folly (or worse) to get into the nitty gritty of part such-and-such-failing-and-how-to-replace-it-and-cost-and-blah-blah-blah UNTIL the fault(s) in a system has/have been definitely isolated.

I rechecked the vacuum and found it to be nearly 15" with only the vacuum pump and brake booster in the circuit. There is a 3 way connection that goes to a brown tube, a green with yellow stripe, and a yellow thing that looks like a check valve or filter of some sort. Off the back end of this comes a yellow line with grey or black stripe. With the key off but engine running, i can make it stop by pulling the yellow line off the yellow filter looking thing, and blocking it with a finger. It takes about 3 or 4 seconds for it to shut down. So hopefully i am on the way to finding the problem, downstream of that yellow /gray tube, which is for the locking circuit (?). Weird thing is that the lock have always been iffy, and are about the same. All these tubes go thru a rubber thing in the firewall, so i am hoping to find it on the other side and see where the break is. Or maybe just block it off, who needs door locks?!


Thanks for all the advice; you fellow Mercedes victims always come thru for me!

15" of vacuum?! Why that's a sprightly vacuum pump on your engine.

"yellow thing" = check valve.
Off of said check valve:
A. Yellow with gray stripe ends at the vacuum reservoir located on the ceiling of the trunk. By having it connected to this check valve, a properly functioning vacuum circuit will retain vacuum when the engine is off.
B. Solid yellow line - the locking sub-system. First stop of this line is in the driver door. From there (for the sedan), it makes its way to the rear left door, passenger door, rear right door, fuel flap, and ends at trunk lock.

Be careful with those yellow lines: One has the stripe and one doesn't. Know with certainty which one you are dealing with at any given moment. I suspect that what you wrote above (if I am reading it correctly) is erroneous on this point. While the vacuum reservoir has been known to be the source of leaks, it is FAR more common for the locking sub-system to have leakage.

In theory, in summary, and based upon what you wrote above: If you take the solid yellow line out of the vacuum circuit at the firewall (read: block the port to which it slides into [so no vacuum leaks]) and the engine shutoffs off properly, the conclusion would be a leak(s) in the locking sub-system (since you have not indicated that the green circuit [climate control sub-system] is faulty). On the other hand, what I just wrote is from my mind, an entity that produces errors from time to time. I encourage you to continue to work with facts as measured.

The door locks will still work without vacuum, just not automatically. And the driver door lock is not automatic in the first place.

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