You can try a 2# brake line check valve for disc brakes to keep pressure, but no drag, on the front lines to prevent sucking air in. A 10# can be used if desired for the rear drum types, since you have a "dual line master" now. Attached is a pic of my "single line master" with 10# check valve for "drums" installed. Since you said you installed disc fronts, I saw an article on "self bleeding valves for disk brakes" you may want to check into. Good Luck!