If you don't see any fluid on that "floor" then the leak is most likely from one of the metal brake lines going to the rear of the car. The lines come off of the ASR hydraulic unit and go down in the area of the steering box and follow along the frame rail. They are protected by heat shields in the area around the steering box and are next to impossible to see. If you have access to a lift you may be able to see them from underneath the car. I would think that if they have to be replaced it's going to be a major job.

Good luck,Robert

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Mercedes Benz Master Tech - Retired