Bet your using a drum brake master cylinder which has residual pressure valve incorporated into the flare seat that the brake line screws up to. Discs only take 2psi while drums take 10psi and the 8# difference will make disc pads drag. I use a #6 (or #8) tap in the center of the flare fitting, screw in a machine screw and pull seat out. Throw away the guts and reinstall the seat then use a 2# residual pressure valve in the line going to the front.
