Power steering not working properly

My truck listed above has bad power steering. It started about three years ago. I had my truck aligned and when I got it back I noticed that it was pulling to the right, ever so slightly, I mentioned it to the mechanic and he told me that he noticed that also and said that it was in the steering gear box, so I bought a rebuilt gear box from Rockauto and installed it. Strangely after installing the new gear box I still have the pulling to right problem. I just lived with it. I have replaced all hoses as well. By the way, wheels are in perfect alignment, had them checked. After about two years of living with this pull to the right I decided to change the pump to see if that would help. I ordered two rebuilt pumps from Rockauto one for my 2003 Sierra and the other for my truck. The Sierra, no problems. The truck, after installation seemed to work at first but still difficult to turn had to increase RPM in order to turn at slow speeds. I ordered a rebuild kit for myself to rebuild the old pump I took off. Rebuilt it, installed it and again it was good for the first couple of turns then it got almost impossible to turn at all, so I bought another rebuilt kit to rebuild the rebuilt one and just finish it yesterday installed it and first 3 to 4 turns felt like I got it right this time, but nope. When I left my driveway this morning it was almost impossible for me to turn the truck unless I was moving. I noticed after about ten miles of driving that as long as I didn't try to push hard against the wheel it would turn fairly easy even at low speed. That didn't last long. I stopped at a customer for a call and when I got back in my truck to leave I couldn't turn the truck hardly at all. It felt like the harder I pushed the harder it pushed back at me. I am completely stumped. I don't know what to do at this point. I have replaced pretty much the entire steering system.