It's a long shot, but back in the early 1980s I had a problem with the power steering on a Kenworth 18-wheeler, and it turned out to be the filter in the pump. Seems there was a batch of bad filters which disintegrated and clogged the adjacent screen. We were adviced by telephone what to look for, and we were able to clean out the gunk in a truckstop parking lot with minimal tools.
Might be worth investigating before springing for a new pump.