I just took my entire PS system down because all the hoses were deteriorated beyond belief. It appeared as though the ZF pump had been rebuilt at some point because the metal retaining spring at the rear of the pump was partly broken.
I find myself shaking my head in disbelief whenevr I find some repair done by the PO. In this case, the retaining clip was mangled and it appeared that he simply used automatic transmission fluid (it was very red in color when I drained it) although it could have been some other type of hydraulic fluid. The bubbling and blistering I had on the hoses suggested that some type of incorrect oil had been used. I mention this because I felt as though I had nothing to lose by using whatever kind of hydro fluid I had on hand. Nonetheless, there is a list of compatible hydro oils listed in the Blue Book shop manual. I ended up using some type of Castrol.
We tested the pump and found it to be producing plenty of pressure. Steering effort seems stiff (even by BMW standards), but no groaning, whining or noises. I have driven other coupes that had lighter effort, but those coupes had been heavily modified and could have had a E28 replacement pump changing the fluid rate.
As this relates to the original post, the PS pump looks as though it would be simple to rebuild (I bet there are many other ZF pumps using the same rotary vanes and seals), but the steering box looks scary to me. I replaced the bottom seal for the steering arm, but beyond that I'd probably be looking for a replacement unit rather than rebuild.
