I have a 2002 GMC Sierra 1500. The brakes do not work correctly. They make an almost grinding noise when braking and then the pedal goes to the floor. I have to pump the brakes to get the truck to stop. I have replaced the rear rotors, all four sets of pads, the brake pump module, and the brake pump and the truck still stops like $*. Should I replace the master cylinder and/or the brake booster?