If the piston is pushed in enough to cover the fluid return hole, it creates a sealed system from the master cylinder down to the calipers. As the fluid heats and expands, it has nowhere to go and starts applying the brake, which creates lots more heat. You know what happens next. The fluid needs to escape through the small return hole in the MC as it expands.
Pulling on your brake lever, you were trying to compress a column of already high-pressure brake fluid.
You really need a dial indicator to check the rotors, but if you ride the bike and don't feel any brake pulsing, you should b