According to MB, the brake pressure for the ASR comes from the pressure accumulator. https://docs.google.com/fileview?id=0B-WRh0Hf7VdZMmE4MTE4N2EtNDJmYi00YjBjLWEzM2MtNmNkMDUwNzYzNTUz&hl=en See pdf p.10

The Return Pump/Charge Pump (A7/3m1) and the ASR Pressurizing Pump (M15) work at the same time. See pdf p.31. There is also a pressure switch involved (A7/1s1).

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