I used to be able to find Vacuum schematics...(or maybe that was only Diesels?)
I think this is the Vacuum System for your model(THINK is the operative word):
http://www.detali.ru/cat/oem_mb1.asp?TP=1&F=124051&M=104.980&GA=722.359501&GM=717.450&CT=F&Cat=15T&SID=80
Your Gasoline model MB gets it's Vacuum from the (or A ) Manifold source.
[If you're hooked up to a line that terminates in the Manifold (With the M.V.)
you may be trying to "Evacuate" the Planet's Atmosphere through your
Engine's Intake]
Better way: Start at the Source...'Find the Line that supplies Vacuum from the Manifold to the rest of the Chassis.
(EXCEPT the central Locking system).
Engine ON,stick "Source" Vacuum line onto nipple of MityVac...'any Vacuum???
Yeah, it's entirely possible a inadvertently knocked off line would be the
cause of your angst.
The other Deceased Equine I keep Beating is...18 to 25 year old cars
[Even Mercedes] have subjected their Under-Hood components to an
Awful lot of Heat Stress Cycles.(All the Little Rubber Connectors are
studying Fresh New Ways to Crack and Fail [Out of Human Sight].)