I don’t think it’s possible to overfill the reservoir and have it actuate the brakes. If it was that then both pads should be worn equally. I just changed my rear brakes at 4500 and they may have went another 1500 but both pads were worn equally. I think the holes in the pad are binding on the guide pins and not releasing. Do you get a soft pedal some of the time? Or does the pedal always feel normal?