Usually the telltale sign of a leaking pneumatic mechanism is the sound of escaping air. I could always hear it plain as day, but you might have to take the interior door panel off to hear a more subtle leak.

D

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1995 E300 146,000 miles "Celia"--sold
1993 300sd 131,000 miles "Konstanze"--with Mom in Tampa
1987 300sdl 225,000 miles "Frau Grau"
1980 300sd 164,000 miles "Old Yeller"--sold