John, I second the recommendation of taking your SL55 AMG to the dealer and having them sort out the problem. Have them supply you a MB loaner and keep your car as long as it takes to resolve the problem. Something is definitely wrong. A rust buildup should not lock up one of the wheels, and you shouldn't have to drive the car after washing it to dry the rotors and brake pads. Certainly the dealer doesn't do that when they wash the pre-owned cars on their lot.

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Fred Hoelzle