Power steering problem

[DetroitDan]

My 87 half ton 4x4 suddenly started binding when turning at low speeds. Checked and fluid was almost empty, filled it no better, gave it a couple days in case it needed to work out it the air. Noticed steering stabilizer was leaking, replaced that. Old one would bind and stick when compressing by hand, thought that was the problem but it wasn't. Finally changed pump and reservoir, and new belt. No better. Re-tightened belt and flushed the really nasty mud out of the system and refilled it. Worked perfect for a couple minutes then the hard turning is back. It's only at low speed, really tough to make a real low speed turn. Also, I checked the front axle shaft u-joints, don't think the bi ding is coming from there. It feels hydraulic. So what's left, the steering box? I've never had one go bad in 30+ square bodies. Is there any adjustment, fix or flush? The ball joints passed inspection, so they're not loose or sloppy, but they're not likely to bind in this manner. It definitely feels like a hydraulic problem, just like when the belt slips, but the belts not slipping. How would I know if the new reman pump was no good? I don't want to think that's the case.
Thanks in advance for any advice.
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