It sounds like you've done everything you can do without dismantling the steering box.
Disconnecting the drag link was just to prove you had free oscillation of the knuckles, which you have.
I have not experienced the following problems personally, but these are my guesses as to what the problem could be.
1) Fractured torsion bar on the input shaft, It allows torque in one direction but not in the other. The spool valve only moves to direct pressure to one side of the piston.
2) Bent sector shaft, with the same bearings it got bent with. It can move sufficiently in one direction, but in the other it is binding.
3) A problem with a seal on the spool valve so pressure is leaking to the return line.
It's just a theory.
Definitely call/email Blue Top. Don't take the box apart while you still have an opportunity for a replacement or refund.