Here's a photo DIY on how to get the pump off and overhaul it. It came from a W140 forum, but it is the same M119 engine you have in your car. I just replaced my pump using this procedure as a guide for removal and it works well. And yes, those back two bolts are a major pain to get off and back on!

http://www.benzworld.org/forums/w140-s-class/1481425-photo-diy-steering-pump-overhaul.html

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles