Brakes are engineered as a complete system- - - -master cylinder diameter and stroke, which controls the volume of fluid moved, and the pressure that's developed, plus the diameter and stroke of the wheel cylinders and/or disc calipers. Then there are such things as proportioning valves and other hardware and plumbing to be considered. To simply swap the master cylinder for a dual line unit without using the entire system it was designed for would require some pretty complex calculations of fluid displacement and system pressure to assure that the brakes would operate as safely as the original one. A drum/drum dual line brake system usually has a MC with equal volume fluid chambers. Those are a little hard to find these days. The last time I can remember owning a non-power dual drum brake system was on a mid-1960's Rambler station wagon.
Jerry
