It sounds like a broken proportioning valve spring.This causes assist in only one direction. Jack up the front of the car (both wheels off the ground)and start it up. If its the proportioning valve the steering wheel will turn all by itself. There was a company a few years ago selling the springs but you'd be better off replacing the steering box with a rebuilt unit.
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 20 Years
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