When the truck is cold, it is very hard to steer, when it warms up it gets better but it still lacking the power it shoul have. belt is new, no slipping, fluid is on full. Dont know how old the fluid is. Could changing the fluid help the problem? Can I use ATF for in the pump?

Yes you can use ATF in the pump. Is hard to turn because the fluid in the steering box is not warmed up yet which acts like a hydraulic/lubricating fluid. It gets better when it warms up.

Its probably a safe bet the pump is getting tired. Is it worse if you step on the brake at the same time?

A flush and new fluid could help and it would definatly be nicer to a new pump.

If it is commonly very cold where you are, I would flush the system with/and run a good synthetic PS fluid like RoyalPurple, AMSOIL, etc. Much better than regular PS fluid, more consistant viscosity.

Check with the Stealer. There was a recall on the power steering pumps. No time/mileage limit. I got the one on my 2000 K3500 replaced for free last year

What years was the recall on?

The recall my truck ever had done was the wiper one. It didn't even get a IP/PMD replaced by the dealer...but someone put one on it out of pocket.

Also, will this recall help fix my leaking PS pump?

Not sure what years it covers but mine is a 2000 K3500 4x4. I asked about my 95 and they said no recall so not sure. I would call the service manager at your local steeler. They get paid to do it from GM so they will if it's on the list. watch if they offer to "replace your belt". The s$umbag called me and said hey your belt doesn't look so good. A new one is 65$ want me to change it ? I said yeah becuase yes I knew th belt was bad and although I might have gotten it a few bucks cheaper at Napa I figured what heck just get it done. well he charged me for the belt and a hours labor to change it. I went nuts. I'm like you s$umbag you had the belt off to do the pump anyway how the hell do you justify that ? I f'in had to pay it to get my truck. I went to sue them later and they went out of business(gee there's a surprise)