SL500 Pwr Steering

[Denis O]

I have a 97 R SL500 which has developed an odd fault with the power steering.
On centre to right lock all works well. On centre to left lock it feels to be void of any power assistance and the steering box hisses all the way.
It's been taken out, sent to an expert company for rebuilding and put back in but no change. It's going back to the expert this week for more work.
Any ideas from you guys as to what the problem is.