Power steering problem

Did this start when you replaced the gearbox or is this why you replaced the gearbox? Have you checked the lines to see if there are any restrictions? Sometimes they can have a collapsed inner liner and the outside of the hose seems fine. I had one where fluid kept pumping out of the reservoir and all the hoses looked great, but when I took them down one was almost completely collapsed inside. Not the exact same issue, but it sounds like your pressure isn't being released. If it started when you replaced the box, are you sure you got all the lines back in the right positions?

 

This is usually a bent torsion bar inside the gears rotary control valve. Are you replacing the gear with reman units are simply having your original one rebuilt?

 

You keep turning until it locks?
What is locking?

 

What he's talking about is the end of the throw where the steering wheel stops turning on either side, like when they say to bleed your power steering system turn the wheel lock to lock. That kind of lock.

Thinking more about it as well - do you have a tie rod going from left to right or a cylinder on the right side or another box on the right side? If it's the type with lines to a cylinder or box on the other side, it could be that the pressure isn't shutting of to them. What happens if you don't let go of the wheel and try to stop it or even reverse and go the other direction? Are you about to easily turn it back, force it back or there's just nothing you can do until it hits the lock and stops itself and then it somehow frees up? Any other symptoms like noise or chatter? Aside from the box and the pump, anything you've inspected and ruled out completely?

Also it's really important if this is the reason you changed the box the first time or something that happened after you changed the box. I mean, obviously the problem isn't the box - no way you got 3 boxes with the same strange issue - but the answer to this question determines whether or not you have to review your work or your original diagnosis.

 

This can happen with really messed up caster, like severe negative caster, but this also comes with really hard to control handling characteristics. Have you had any other suspension or front end work done recently? Why was the original steering gear changed? If you jack up the front end so the steer tires are off the ground, with the engine running, if you move the steering wheel and then let go, does it continue to turn on its own until it hits the stops, or is this just when you are driving it?

 

Well let's see.
Power steering is simply a hydraulic pump, pumping a hydraulic fluid threw lines to move a shaft.
It is a stead flow and has a releaf values to dump the pressure back to tank and then back to the pump.
If any of the pressure releaf values are not working correctly or cloged, you will then have a one way hydraulic ram moving only one way direction and not able to release the pressure off the line to go back the other way.