Here's my considered opinion as to what is happening. Of course, it's just a guess. When I picked up Rosie (Rosso Corsa) from Alabama, the rear rotor was blue and cone shaped. (!!) It sounded like a gun went off when I loosened the last rotor bolt. That sucker had been HOT. A guy that used to ride with the Distinguished Previous Owner said he was always riding the rear brake, because you could see his brake light a lot. The brake line was poorly routed, and burned through.
Had a new brake line made up, bought a new caliper and rotor. I don't remember if I rebuilt the M/S or not, but probably did. Went for a ride around the mile square, and the new rotor was hot. Hmmmm. Let it cool, rode it around the square again being careful not to touch the brake pedal, and it was hot. Hmmmm some more. Decided that maybe if all the air wasn't out of the line it could get hot, expand and push the piston against the rotor. ?
So, I hung a machinists vise from the brake pedal overnight, and the next day the pedal was hard as a rock.
The rotor never got hot again. Take this for what it's worth..