Front disc brake conversion questions.

[425HP409]

You have to use a master cylinder that is built for either all drum, disc-drum, or all disc. The outlet of the master cylinder for drum brakes has a check valve behind the tube seat for the line. This is to hold a little residual pressure in the drum system as the shoes are not in contact with the drums and keeps the wheel cylinder cups sealed against the wheel cylinder and helps the brakes apply quicker when needed. The discs do not need, and should never have this check valve or you will have residual pressure applying the pads to the rotors at all times.
Back in the day when we rebuilt master cylinders, the kits came with new check valves and seats which were replaced.