Isuzu PAS pumps are just a vane pump with a pressure relief/regulator valve. Not much to go wrong. I had an issue with mine where some crap inside the pump had jammed the relief valve open so the steering would go hard intermittently. Eventually it got stuck in the valve and the steering was hard all the time so I pulled it apart and found the issue. The crap in mine was from it sitting open on the shelf at the wreckers for ages. I rebuilt it before using it but there was still something inside which dislodged when it was operating.
Unlikely to be the issue with yours unless you've had the system open.
Something else to to check are the steering shaft universals as these can bind up.
Maybe redo the lock to lock test you did with just a bit of weight on the wheels as it could be a case of a little load causes it to bind up harder.
Also check for anything physically rubbing on the steering components - especially with weight on the suspension. IIRC someone had a snorkel hose catching on the steering shaft recently.
Steve
1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
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