Under floor masters are a real B**ch,,,,rent yourself a pressure bleeder,,,,,and like everyone else is saying,,,,,bleed out the master first,,a quicky way to do this,,,have the wife or girl friend pump the pedal,,while you get underneath and crack the rear lines first,,then the fronts on the master cylinder,,,,tell the wife/girlfriend/buddy,,to really step down and hold it there,,,,you want to open the lines on the downstroke,,and then tighten before the upstroke.( you only have to do this for two strokes)
Also,,,,the proportioning valves also act as a pressure resevoir for the front discs,,(dont forget,,the fluid has to travel uphill before it reaches the front disc brakes,,it might be a good idea to open ahead of the proportioning valve on the downstroke too.),,,,,,,after you have bled the master cylinder,,,then attatch the pressure bleeder to the system and bleed away.
attach rubber hoses to the bleed nipples and route them into small pop bottles,,to save the fluid, ( warning!!! do not re-use afterbleed fluid for a period of 24 hours until all the airiation is gone) persistence pays off in the long run.