They started using road salt or Mag as it's called in our state about 3 years ago. Our 2004 F-150 started getting a bad break shake. Thought it might have been a warped rotor, but could not feel any pulsing at low speed. From the outside, the disc and pad looked perfect. I pulled the brakes apart and found the back of the rotor badly gouged. The metal piece on the rear of the brake pad had rusted away in places and the piston caused the pad to push unevenly on the rear of the pad. The clip that holds the pad in the piston was also badly rusted and one of the 3 fingers had broken off.
I ended up replacing all 4 discs and pads. The rear also was causing a shudder when braking and also had parts of the metal piece on the back of the pad rusted away. Hate the "Mag" !!! The local Ford dealer said they are replacing a lot more rotors as a result of the road salt.