Just purchased 93 850 that developed brake grind/squeel shortly after purchase. Took it in and was told I needed front/rear pads and front rotors, this work was completed. Squeel was still present only when backing the car up and applying the brakes. This time went to my regular mechanic who said that the back rotors were rusted and too bad to repair, this work was completed. Since the back rotors were replaced I have had the brake pedal pop back toward me twice when I applied the brake at a speed over 45 mph....is this normal ?? It did not do this prior to this brake work being done.