Brake Master Cylinder problems of the 2013 Kia Motor Soul

The vehicle has been making scraping noises when the brake is applied. My mechanic assumed it was the front brakes although it only has 14k miles on it. The front brakes and rotors are fine but the back rotors have grooves through them and the brakes themselves are mottled looking. Concerned that this is a mechanical problem and the car is still under warranty, the mechanic told me to contact the dealer. I took it to the dealer who said #1-he could not hear the scraping and #2-the brakes are fine with plenty of wear left on them. When I questioned why the back rotors would wear before the front and why they would be worn at all with only 14k he had no answer. Since there was enough wear remaining on the rear rotors there was no action required by them. Master cylinder? e-brake?.