There is another large O-ring where the front of the pump mounts to the rear. When I pulled mine out, that ring seemed to be fine. It didn't look like something that would be subject to leak excessively, but that O-ring is included in the rebuild kit. BTW, if you order the kit, look at the metal tag on the pump first and get the type. There were two different brands of pumps used on that car--a ZF and a Vickers. I think the difference was that the Vickers reservoir was not an integral part of the pump bracket, whereas the ZF reservoir is welded to the bracket and comes out as one piece.

Here's a link I previously posted with some pictures of the inside of the pump. http://www.peachparts.com/shopforum/showthread.php?t=192545

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