My Engine Service Manual covers the Model 115.1 engine. It shows a picture of the power steering pump with a bolt near the top and one near the bottom. The instructions are:
1. Loosen fastening screws (1 and 2).[These are the bolt referred to above].
2. For tensioning V-belt, push power steering pump outwards by means of a lever.
3. Tighten fastening screws (1 and 2).
Perhaps it is just lodged in place and is going to take a bit of prying to break it loose.
I realize this may not be much help, but it is what the MB manual prescribes.