I guess that it's already been explained that the PS pump supplies the assist to both the steering and the hydroboost brakes. It's very important to use the right oil in it and bleed it completely, other than that, it's a pretty simple system.
You say that you like to change out parts before they fail; I hope this doesn't mean that you are changing out perfectly good, functioning factory parts for replacement parts just to create problems in your life and forum threads(?) The parts that came on that truck are ALWAYS going to be better than anything you can buy 14 years of globalization later.
Was the steering box bad that you replaced? I have a Redhead box, (that sounds dirty), sitting in the box waiting to be installed on my 230k mile 1999 Ford F-250 but the original box is junk from someone over tightening the adjuster before I got it. Same w the PS pump, was it failing?
DanJ, a poster here has a 2006 Duramax dually that he bought new w 300k? miles on it that he's never done anything to other than brake pads and fluid changes. Says it's the best piece of machinery he's ever owned, period. And he is way more than qualified to replace parts on it if needed.
At this point, start w draining and refilling it w the correct, GM fluid. I've never understood people who spend all kinds of $$ and time changing parts and then get cute w the lubricants...trying different products off the Autozone shelf. Use the right oil, period and have an easier life. That's my experience, anyway. I've only owned about 150 vehicles so it's a small sample but still.
I told you from the start that when you bought the truck that you need to find a wrench who really knows that particular rig. Places that say, "All Car Specialist" suck for diesel PU trucks.
