1972 duster 360 904 8 3/4 391

Hi Scott here, I need some help my friend and I just completed a front drum to disk break conversion. The hole kit including all the break lines and the master cylinder. We even changed the rear from the small bolt pattern to the larger bolt pattern and larger Drum breaks, including new axles. The problem I am having is the breaks are mushy. The pedal goes to the floor two time before gaining any breaking power. The only thing I did not change is the proportioning valve. Nothing is leaking what could I be missing? Thanks Scott USN Retired.