Many factory master cylinders have a residual valve under the line seat for drum brakes. The residual valve keeps a small amount of line pressure to prevent the wheel cylinder from sucking in air upon release of the brake pedal. If the residual valve is present and residual pressure exist then it will keep your disc brake caliper partially engaged, burning up the pads and rotor.

You can remove the valve by removing the brass seat with a wood screw and reinstalling it without the valve, or you can replace the master cylinder with a unit without residual valves.

The relative size of the primary and secondary pistons in the master cylinder to the size of the caliper piston will determine pedal effort and travel. A relatively smaller diameter master cylinder will require less pedal effort but more pedal travel.