Drum brake master cyls generally have a non return valve behind the brass cone in the m/c outlet. If you convert from drum to disc on either or both axles you must lose this valve (spring and split rubber cup.)
Disconnect the appropriate brake line, screw a self tapper into the cone and using a pair of side cutters pry it out. discard the valve, refit the cone and bleed brakes.
The valve is only there to maintain fluid at the wheel/cyls - calipers don't have return springs.