IH 5288 hard steering

A neighbor has a IH 5288 and it was steering hard to no steering at all. The filter was checked first and we realized the filter bypass valve was missing. So immediately thought that there was garbage in the system, I figured that parts were to be taken off to be cleaned and inspected. A replacement bypass valve and filter was installed and new oil put in, then I started with the hand pump and sure thing the shaft was scored, you could barely turn it by hand. So we flushed out the lines and replaced the pump with a rebuilt one. The tractor seemed to steer a bit better, but with a little extra effort while moving and no steering at stand still. I checked the troubleshooting section of the manual and all it suggested was either mechanical binding, which was ruled out when moved the wheels manually back and forth with a bar, the priority valve or the pump. I have another 5288 in the yard and you can steer it with your pinky, so we started to swap parts, we changed everything around one by one except for the handpump, every time the steering improved but never as good as my tractor. And on the other side there was no change whatsoever on my tractor. My neighbors tractor improved about 80% I would say, but when I drove it back to his place after the oil warmed up I lost all steering, the steering wheel moves but not the front wheels. Any suggestions will be appreciated

it sounds to me like the pump that supplies the power steering system is sucking air.

Thanks, I'll check that out