I have no rear brakes

Check the backside of the rotor for wear. A common issue is for the caliper slide pins to seize in the bracket, preventing the caliper from floating and self-centering. As a result, only the piston can push the pad against the rotor on one side - the inside. So the outside will look brand new, but you are only pushing on one side of the rotor.

If that checks out, crack a bleeder on the rear brakes, connect a hose to the bleeder and put the end in a bottle. Have a friend press the pedal. If fluid freely flows, then it's likely your rear calipers are seized and should be replaced.