To be safe, have the brake lines checked for leakage. I had a BAS/ESP issue last year on my '01 E430 because an improperly installed brake line ruptured and leaked much of the fluid.

This contributed to a BAS/ESP issue because the loss of fluid caused the brake pedal to travel closer and closer to the floor, beyond the apparent limits of the brake switch plunger, so the plunger got stuck in the depressed position -- and remained that way for almost a day until I heard a metallic "ping" (the plunger unsticking itself) when releasing my foot from the pedal.

By then the ruptured brake line was discovered, resolved, and the problem has never recurred.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black