I have a 2008 Silverado 2500hd that will only go about 20mph and almost stalls when I hit the brakes I’ve replaced spark plugs and wires. could it be a brake booster or a iac valve please send me in the right direction to fix it

 

You need some OBDII codes to help us. The part-stores will do it at no cost. Write them down and post them here. Likely a fuel system issue of some sort.

 

I don’t have a scanner does auto zone rent them out or am I going to have to buy one

 

No. They will scan it for free. No need to buy a scan-tool.