Vacuum leaks are a pain in the arse but can be found by isolating the various circuits. First thing you need to do is find a Vacuum diagram for your vehicle on this or another website...

Each system works independantly of the other but all are fed by the same pump. For example, a leak in your doorlocks wouldnt necessarily make a leak in your brakes. Heres where to start...

1. Check for vacuum directly off your pump (I believe this works because you can STOP!)

2. Remove the recieving end of a vacuum line to a component and check the vacuum (for example...take off the shut-off valve line and check the vaccum pressure there) do this to as many systems as you can access comfortably and see what your readings are.

3. Ok, so everything is normal vacuum wise...lets move on to the actual components... I had a stop-valve go bad...so many times, these delicate diaphrams can leak...make a little vacuum pressure on a component and then see if it holds the vacuum...if it doesnt...leak!

4. Also, look for what we call "red flags" in the system...these are lines broken, cracked, or downright missing. Over the years, they can dry out or be knocked off and broken...look for that before condemning parts.

5. Want some vacuum line but dont wanna go to Merc? Take a piece of 10 ga solid copper wire and strip the sheathing off an individual strand...it fits perfectly as vacuum line and stands up to the heat (did it on mine...10,000 miles later...still good!)

Do that and tell me about the results...remember, the Mity Vac is INVALUABLE for a diesel mercedes!