Steering Problem on M51A2

I am having a steering problem with my M51A2. Here is what I have discovered so far. With the front wheels off the ground and the engine not running, I am able to turn the steering wheel to the left just over 1 3/4 turns at which time the left wheel hits its mechanical stop. I can only turn the steering wheel just over 3/4 of a turn to the right. At that point, the right wheel is nowhere near its mechanical stop. In both directions the steering wheel moves easily. If I start the engine, I get the same results in both directions.

If I put the front wheels on the ground and start the engine, I can turn the steering wheel to the right approximately 3/4 of a turn. It is easy to turn after some initial resistance. Turning the steering wheel to the left, with the engine running and the wheels on the ground is virtually impossible. It's as if the power steering is fighting turning the steering wheel rather than helping.

I am guessing I have a control valve problem with is causing the initial resistance when turning right and the large resistance when attempting to turn left with the engine running. What I don't understand is why the steering wheel turns all the way to the left but less than half way to the right. This would seem to indicate a problem in the gearbox with my worm gear or sector shaft or possible a problem with the assist cylinder that keeps it from moving its full travel.

Does anyone have any ideas on this? I'm about to buy an new steering box for my truck.

Have you tried to center the steering gear/system? this means separating the gear from the linkage and turning the wheel in one direction to full stop, then counting complete turns in the other direction. You then half the number of turns back to the middle and your gear should be centered. then hook the linkage back up and adjust your stop bolts to yout\r housing. Please read the TM for the correct specs. Hope this helps.

Follow Kredds advice there, I had that problem with my fire truck after We fitted air power steering, The 'helper' had not centred the steering geometry.
So, I disconnected the drop arm and counted the full turns lock to lock.
The counted half way back.
centred the wheels to the steering wheel,
Assembled the unit, tried it turn to turn, It was equal, so It was (and is) good to go.

Can anyone tell me how many turns lock to lock a M51A2 should have? I have looked at the maintenance manual and operators manual and don't see this information spelled out anywhere.