I've been using this one for over a decade and it works great.

http://www.samstagsales.com/mercedes.htm#brake

UKKO
126-00 000 589 52 43 00 Brake Piston Reset Tool, used to spread calipers to allow room for new pads. Fine quality tool, Made in Germany! Current replacement for 111 589 07 37 00 and 123 589 00 37 00

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