I'm guessing that the rubber seal in the vacuum booster accumulator has hardened and it isn't working smoothly. I rebuilt the one on my 59 220S years ago and it was very easy but don't know if yours is the same.

Reid

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Reid
Past ('59 220Sb, 2-'59 190's, '92 300D 2.5T)
Current
'04 E320 4Matic Wagon
'00 E320 4Matic Wagon