Early A dual reservoir conversion

You don't mention which dual master cylinder you used- they are not all created equal. There are different bore sizes, and later disc masters have reservoirs with larger capacity for disc brakes, as well as the use of residual valves in the MC circuit. There are also different proportioning valves/distribution blocks between disc and drum systems.
Each of these makes a difference in the operation of the system. We need more information on your setup.