I have '77 240D which I purchased new and have kept well maintained.
It will not maintain vacuum when doors are locked. If doors are not locked, it will maintain vacuum indefinitely.
I have:
-Replaced vacuum pump diaphragm
-Replace line including check valve from vacuum pump to system
-Checked blue and black check valves under hood-they hold
-Replaced three-way valve in driver door
-Replaced check-valve in yellow line between vacuum source and center of three-way valve
-Checked diaphragms in all three other doors-discovered no leaks
-Put all-night test in vacuum tank-it's ok
-Tested gas-flap and trunk diaphragms-all ok
-Replaced three-way and four-way vacuum fittings under hood
-Vacuum pump has 22 inches of vacuum
I have checked everything I know to check and have been unsucessful. What am I missing?
My mechanic and Mercedes dealer have been unable to assist.
Is there a trouble-shooting guide available?
Would the manufacturer's cd manuals which are offered for sale be of any assistance?

More than likely, You have a pinhole in one of the door actuators that is not showing up. You Have to check them with the door locks in both positions and with low vacuum levels. I have seen the reservoir leak. Just a thought.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

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Originally posted by walt:
I have '77 240D which I purchased new and have kept well maintained.
It will not maintain vacuum when doors are locked. If doors are not locked, it will maintain vacuum indefinitely.
I have:
-Replaced vacuum pump diaphragm
-Replace line including check valve from vacuum pump to system
-Checked blue and black check valves under hood-they hold
-Replaced three-way valve in driver door
-Replaced check-valve in yellow line between vacuum source and center of three-way valve
-Checked diaphragms in all three other doors-discovered no leaks
-Put all-night test in vacuum tank-it's ok
-Tested gas-flap and trunk diaphragms-all ok
-Replaced three-way and four-way vacuum fittings under hood
-Vacuum pump has 22 inches of vacuum
I have checked everything I know to check and have been unsucessful. What am I missing?
My mechanic and Mercedes dealer have been unable to assist.
Is there a trouble-shooting guide available?
Would the manufacturer's cd manuals which are offered for sale be of any assistance?

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This falls under the how did I miss that catergory. There was a pinhole leak. Fortunately, I found it in the first one I tried. Thank you for the advice.