Okay, that eliminates the easy stuff (disconnected tre, jammed log up in the works, etc). Sounds like the pump and linkages are okay.
The cheap, easy, first effort would be to flush out the old fluid, and replace with new. Some ps systems have little orifices within the steering boxes that allow fluid to flow, and they can get plugged with stuff, like PTK said.
I'd stop short of compressed air, but me being me, I'd disconnect the hoses at the steering box, pump a little kerosene or ATF through the steering box in both directions with a hand pump or turkey baster. Bleeding the air after the exercise may be a challenge.
If a fluid flush doesn't knock it loose, it may need a teardown.
Wait for other comments, I've done exactly one steering box in my life, in 1983 or so.
