Deep grooves on upper part of rotor

If its not a stone it could be pad material has dislodged from backing plate... Or just worn out

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Shouldn't need to disconnect battery but move keys to back of house as not to activate brake servo pump.

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After looking again your going to need pads.. That looks worn out.

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83K miles? Our 2011 Prius hatchback with over 130K is still on the original brake pads, though most of the miles were highway/freeway miles.

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Pull the wheel and you'll probably see there is no pad material left.
And the top of the pad is metal on metal.
You'll needs pads at the very least, but rotors should also be replaced.
You'll need the tool to rotate the caliper puck back into the housing.
You can rent it from autozone or other auto parts place.

You do not have to disconnect the 12v battery. Just do NOT open the drivers door.
And might be a good idea to keep the key fobs at least 30+ feet away from the car.

 

Brakes must ve been draggin for the longest time with 20 mpgs average!

 

Did you have the rear brakes serviced not too long back? If the caliper pistons aren’t rotated just right, and well seated thus, they’ll cause grossly uneven pressure application to the pad back, beveled pad wear, area of the rotor not making contact with pad, symptoms like that.

there’s a link in my signature on rear brake drag; if on a phone turn it to landscape to see signature.

 

Had you ever flush the brake fluid?
Or cleaned the glide pins?

 

Where in the States are you, more specifically? Is there road salt usage?