I believe master cylinders for discs have a larger reservoir for disc brakes.
some good info on drum brakes, disc brakes and master cylinders
http://www.piratejack.net/Tech-Support/FAQs-Master-Cylinders.htmlCan I use my drum brake master for disc brakes?
No, for two reasons. First you will not have enough pressure or volume and second the drum brake master will have a residual valve that will cause the disc brakes to drag.
