80300dbenz,
Sounds like maybe you have a leaky reservior. There should be another line in the engine bay that goes to the reservior. Mine is brown I believe. Pull vac on that and see if it holds. You may have a bad connector on the line that goes into the reservior. If that doesn't hold, find the connector and plug it and then pull vac again. Or just replace it. I would replace all of the connectors. They are a huge source of vac leak, and they are 25 years old.

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