The220D, a quick check for fluid in the brake vacuum reservoir is to pull the brake master cylinder away from the vacuum reservoir and look: 1) is the rear of the master cylinder wet? and 2) make a dipstick out of an old coat hanger and stick it into the vacuum reservoir. Should come out dry in a perfect world. Brake fluid is a good paint stripper so use lots of rags/paper towels. If both checks come out dry then perhaps you have a pinhole in the vacuum reservoir. A hand-operated vacuum pump is a handy tool: you can connect to the vacuum pump inlet hose (after reconnecting the brake master cylinder) then pull a vacuum on the hose and vacuum reservoir. A small leak is OK - that's what the car's vacuum pump is for - you should be able (with the engine off) to push the brake pedal, push the car to release the brakes, and repeat at least two times before losing vacuum.

When to fix a leaky brake master cylinder? Ask the fellow in the mirror:
1) can he stop WHEN the kid chases the ball into the street?
2) how often does he want to dive into the vacuum pump?

I'm "away" for a few days, but I'll check back to read of your success.

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daBenz - 1970 220D