Chevy truck

Hi I have a 2009 Chevy Silverado hybrid two mode 1500 4x4 I have replaced all of my brakes and the back brakes keep going bad and grinding and I do mean I replaced everything I replaced brake master cylinder (new) calipers on all 4 rotors on all 4 and brake hoses on all 4 can some one please tell me why the back brake keep going out and grinding I only got 3,000 mileson the back of the truck

Hi I have a 2009 Chevy silverado 1500 hybrid two mode 4x4 I have replaced all of my brakes calipers roders hoses all of them a my brakes master cylinder to so why does my back brakes keep grinding what can it be