I have a 1997 F350 Diesel and I have been having issues with the Power Steering for the past 3 months. The local Ford Dealer has replaced the steering box twice, the power steering pump twice and I have had the bushings and bearing replaced in the steering column. The last warranty replacement included the new pump. The steering becomes more difficult as you drive (I think the fluid heats) and the local dealer is using Mercon as the fluid in the system.
Is it possible that the Mercon is not capable with the o-rings and seals in the replacement parts. Replacement parts are from NAPA.