1999 Volkswagen Passat Power Steering Fluid

I have a 1999 Volkswagon Psaat that the wrong fluid was put in the power steering compartment. The mechnic flushed the system has replaced the power steering pump, replaced the racken pinon steering and all the lines but the problem is that the power steering fluid is getting hot and causing the it to whine. Can you please give me what you believe the fix would be. My car has been in the shop for four weeks and I am beginning to feel like it is a lost cause.

Thank you
Gayle Blair