I have a friend with a 92 500E and he is having a noticeable problem with his brake pedal sinking significantly toward the floor as he starts the car. Maybe it is just the power of suggestion, by I noticed that mine is also sinking on startup. I suspect it has something to do with vacuum, but is some sinking normal (of course, I'm pressing on the brake when I start the engine). What could cause this to occur? Any ideas?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles