Grooved Rear Rotor Mystery
 

I have a 2005 Trailblazer which I changed the rear rotors and brake pads about a year ago. I cleaned and greased the pins and they moved freely. I cleaned the pistons and the boots all looked fine and the pistons moved freely as well. But I did have some corrosion on the end of the piston on 1 caliper which actually caused raised bumps on the end face of the piston which presses on the brake pad. I filed and sanded the piston end flat and smooth so that the whole piston would contact the brake pad evenly.Now for the current problem,I have about 7,000 miles on the truck since the brake pad and rotor replacement and both rear brake squealers are squealing. I took off the tires and inspected the brakes.The pads are fine (almost look new) but the rotors have worn down tremendously to the point where there is a deep groove that the pads fit in but the squealers which sit just beyond the pads are hitting the rotors causing the squealing. Anyone know why the rotors are so badly grooved yet the pads are fine.Do I have a problem with the calipers and should I just replace them.Obviously, I know I need to buy new rotors but I do not want to have the same problem occur to the new rotors and need to fix whatever caused the rotors to get an 1/8" plus groove with just 7k of driving.Thanks for the Help!!

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usually the cause is debris from front tire throw off getting lodged between the pad and rotors. you drive on gravel roads?

Very rarely do I drive on any surface that is not paved. And if I do it is usually not for more than a few minutes. I live in northern NJ and all roads and parking lots are paved not many dirt or gravel roads around anymore.