Your rotor may be warped.
Take your wheel off and spin it and try to see what is rubbing on what.
High heat can cause the rotors to warp.
1 more maybe even more common problem would be the caliper is sticking / needs to be honed out and a rebuild kit put in. They can be if not pitted honed out with emery cloth and reused. Most people just buy new ones.
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Save gas
Ride a Mtn bike for errands exercise entertainment and outright fun
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