Anormal brake wear

I own a 2015 Mazda 3 sport 2.0(skyactive). I'm working on changing the brake pads and noticed some abnormal wear. The rear brake pads are unevenly worn off and the rotors is severely grooved. I got new rotors and brake pads but I can't compress the caliper piston. It's one that needs to be screwed in. I use the universal box that I've put on a ratchet and I'm trying to screw it back it, but it just spins and won't compress. I'm hoping ai

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Hi, Alex; Yes, the caliper piston must turn, usually with a bit of down-force at the same time. There is, of course, the possibility that the caliper is 'frozen'. That would also explain the uneven wear. Reman calipers aren't very expensive, and some come 'loaded' with pads. Each time you use your e-brake, the pistons rotate outward to take up any slack in piston travel, but not overtighten.
Good luck.

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Sometimes the piston can get a small bit of rusted spot on it, or more often the cylinder it fits in. That can cause it to stick, often while against the brake disc. If that happens, and in the rear, you may not smell the burning like from front brakes. Removing the piston and rehoning the cylinder, if the rust spot isn't too large, sometimes that part can also be slightly brazed and polished down. Most often today they just opt to replace it.

If there is no rust involved inside the cylinder or between it and the piston, you can use a large "C" clamp to press the piston back into the cylinder. That's what I do, and open the bleed valve slightly at the same time, to aid any air removal in the brake line. Close the bleed valve before releasing the C Clamp when piston is fully back in the cup's cylinder.

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