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Is there anyway you can have a friend mess with the steering while you are watching the pump?
As far as I know the pump is completely mechanical. That means that for the steering to be causing the bog down it would meant that either there is a problem with the belt or the pump is begining to freeze up. OR... There is a clog in the lines (possible?) that will not allow the pump to turn correctly, thereby slowing down the engine? All hypothetical...
Check the belt, is it worn?
I've "blown" up several PS pumps on my '83. Everything from offroading to plain forgetting to check the fluid (I.e. bad maintnence)
Anyhows, if you can see what's happening with the pump it'll help to narrow down the problem. Make sure the pump is turning correctly when your friend is checking the steering. Try checking out the lines that are hooked up to the pump
If the truck is EFI, it should have a pressure switch on the high pressure side of the pump, or in the line, to tell the computer to up the idle. If this switch is bad, the idle will not be increased, and the engine idle can be dragged down far enough to cause a stall.
Go to agrsteering.com Most 4x4 part stores also carry the line and the pump can go for as little as $120.00
If you got the cash and want to restore your steering to it's full potential then I would get the pump and the box.