With a new iron master cylinder, it could be rust/corrosion in the end of the cylinder keeping it engaged. Like what I described above for the aluminum cylinder. Also, if there is an adjustable pushrod for the brake pedal to M/C connection, it could be too long, doing the same kind of thing when everything gets warm and heated up. When the brake fluid gets hot, it will expand and push back up the line as well. I would agree with chasing all the possible causes before replacing something.
Mike Miller