Wierd Jeep steering problem

Helping a friend out with a CJ8.. Problem is this- steering to the right you have good power steering. Steer back to the left and once you get past center there is no power steering. Steering by armstrong, so they used to say. He has already changed the steering box out for another one, and the problem is still there. Don't see any linkage binding anywhere. Steers fine with front end jacked up, put it on the ground and here's the problem again. Any ideas?

I had a similar problem that was caused by a line blockage. Once I corrected the issue I was able to get proper pressure and turning assist was returned to the "left" turn.

My blockage was on a hose between the cooling element and the pump.

Its pretty simple. Jack the wheels off the ground. Disconnect the end off the pitman arm. Start the motor and see how it works. Its either the box or a bind in the steering linkage. Could be the stabilizer shock with the internal valving gone bad. Its not rocket science.