I recently replaced my transmission vacuum modulator because it was not holding vacuum. Instead of throwing the old modulator away like a normal person would, I had to satisfy my curiousity by cutting it open. I found that the interior was severely rusted. (My car is quite free of rust in general and has never been driven on salted roads.) I suspect that a crack in the rubber cap had allowed water (or humid air) to enter the modulator and cause the corrosion. I had assumed that the cap was nothing more than a "dust cover." But further examination suggests that the cap is critical to keeping the modulator air tight. So if your modulator cap has been on your car for a couple of decades, replacing it with a new one might prevent future transmission problems.

for those interested the mercedes part number is
126-277-00-81 old style modulator
and 126-987-07-45 and 013-997-23-48 for the new style, the 013 number is for the o-ring needed for the cap

Was it a lot of work?

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