I had a similar problem with my 1988 190e 2.3 and it was the vacuum hose that comes out of the master brake cylinder. The car ran fine after I replaced it. I found a used one at the local wrecking yard for five bucks.

Check all your vacuum lines.

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I have never met a man so ignorant that I couldn't learn something from him. -Galileo Galilei, physicist and astronomer (1564-1642)