Did you bench bleed your master cylinder prior to install?
Failure to do that, all the bleeding in the world won't make the pedal firm....

If you have bench bled it- you might consider running out to a parts store and getting another front Disk/rear drum master cylinder- I went through 3 the last time i had one go out before i got one that worked properly... And i have heard similar stories from other folks who have done disk swaps... Could just be a faulty master cyl.