Did power steering conversion and doesn't work!

[straight-shot]

I did a power steering conversion on my 70 f250 4wd and am having problems. I used the power steering pump and steering box from a 1970 2wd. Put new seals and gaskets in pump and box.
If I turn the wheel very slowly the power steering works but if I try to turn fast like turning into a parking lot, no power steering. It's like the pump can't keep up or something. Bled air by turning coast to coast many times.
What could be the problem?
Thanks in advance!

[Ranchero50]

Box or pump could be bad. After running it for a while check the coolant. If it's frothy you still have air in the system and since air compresses, the pump isn't going to be able to build pressure like it should.