Tractor Repair: 6600 ford tractor, power steering pump, salvage yard

Question
I have a 6600 Ford Tractor, The problem I have is that it will turn to the right no problem at all.  When need to turn left it has no power steering.  Can you give me any Ideas what to check.  I have already replace the power steering pump and it did not change anything.
Thank
Debbie

Answer
  Hello,

  There has to be a problem in the control unit behind the dash, that is operated by the steering wheel.  If it was the pump, you wouldn't have power steering in either direction.  You might be able to find a good used control unit at a salvage yard.

                       Arnie