I have replaced left front caliper, installed new master cylinder ,installed new power booster, and replaced all the brake pads, when not running the brake pedal is pretty stiff when I start the truck up the pedal seems to be soft and then gets hard towards the end and that's when the brakes barely stop the vehicle I have to push hard don't know what else to do. would the ABS might cause the problem I have no brake or abs warning lights
It sounds as though the entire system may not be bled properly. The vacuum booster is fighting with the spongy feel of trapped air.
I would start with a thorough bleed, including the ABS activation (which always seems important on Chevys) and see if the problem changes.