Checking vaccum on '91 300CE

[jmerc6]

The floor vents on the air/heating system are not working. I suspect a vacuum problem. My cd manual describe the process of checking the vacuum on the flaps. The directions describe the use of a tester which I don't have. My question is can I check the vacuum with my Migty Vac? If so, what should the reading be?
Thanks

[compress ignite]

You're looking for the "Vacuum Pod's" ability to hold a vacuum and not leak...
If it'll hold "15" on the Vacuum side it's O.K.

[jmerc6]

Thanks for the response. I'll check it today.

[jmerc6]

I've checked the main line running to the manifold on my 91 300CE, but got no vacuum. Am I doing something wrong? Or is this an indication of a problem with the vacuum? If so, anybody have a suggestion what it might be?

[compress ignite]

Are you saying you've hooked up the MityVac to the "Main Line"
(I.E. Vacuum Source) [for your HVAC] WITH THE ENGINE RUNNING and
showed no Vacuum?

[jmerc6]

When I say the "main line" I'm speaking of the line under the hood that branches off into three lines that look like they run through the fire wall. It has a black stripe.

[compress ignite]

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].)

[jmerc6]

I plugged one line of my Migty Vac into the green/yellow and the other in the check value, but I couldn't evacuate the line. Does that mean there is a line loose somewhere? I rebuilt the cylinder head and put new head gasket about 3 months ago. Could it be possible I missed a line somewhere?