It could be anything from a defective brake caliper, to a defective flexible brake hose connecting the calipers to the truck. Have a qualified repair shop do your repairs, remember, your brakes are the only thing between you and the guy in front of you! Good luck.
It's probably because you never put them well. It could also be that the kind of brakes that you replaced are not supposed to be fit in your Chevy or they were not of the right size. You need to take your Chevy to a mechanic so that he or she can check what the problem is and fix it.