Reducing PS pump pressure

I have a 98 dodge ram 3500 cummins truck , and I swapped out my power steering pump with a new or rebuilt pump from advance auto parts ( junk ) ...
Now i seem to have low flow or low pressure ,,
I can't turn my wheels when i stop on a rough concrete surface or when I'm in four wheel drive in a bind,, i do have a little bigger wheels on my truck, but the old pump didn't have this problem it just leaked bad....
,,, I didn't know about the pressure regulator in the pump,and i think i need to up my pressure,,,..
So if I take a shim or use a smaller shim will that help???
I found a kit for about 30 $ http://www.summitracing.com/parts/MCH-P306/
I don't think I need to get the kit, if all i need to do is change a shim size,, i think????
please tell me what you think!!!!!