It may be that the PS was not maintained properly. There is a small, donut shaped filter that sits at the bottom of the PS pump reservoir and it is scheduled to be replaced every 30k miles, if memory serves. Typically the PS fluid is at least partly changed at the same time. Check the archives and you'll find what you need to know. It is a simple, cheap service that may restore proper steering feel to your SD. What about the front tire pressures?

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Richard Detoy
'84 300SD
'76 Moto Guzzi 850 Le Mans