My fathers 1998 E350 PSD 275K comes to me with hard steering, I test drive it's definitely not right, you have to steer with both hands. Fluid is full and belt and tentioner 3 months old and tight. I replace the pump and steering box with rebuilt from AZ.
Test drive steering is better but not fixed still have to use your palm to turn the wheel not your finger, I told him to drive it.

Three weeks later back again after about a minute on the road no power steering at all the wheel turns about 2" then it stops , with two hands you can force it. Pull over shut truck off then restart works fine. At idle PS works fine. Sitting on pavement you can turn it at idle from stop to stop with no problem so I am not suspecting the belt. Jack front end up works great with or without engine running.
Clogged return line? At 65MPH it's like driving a old U-haul. What are your thoughts. Thanks Channing