I have replaced power steering pump and rack and pinion assembly and it is still hard to steer.

Hi had this problem several times at the ford dealer. Here is what ford had us do. Sit in the drivers seat engine running turn wheel all the way to the stops put pressure on while at the stops same thing both directions go to the stop hold it under pressure as long as it takes you to count to 5. Do this until all air has bled out and steering shudder is gone.