uvastang

Ive got a 93 mustang fox that i upgraded the suspension and brakes in. I bought used front calipers from a sn95 and have the fronts hooked up and ready to go. Lines and spindles bolted up and fit perfect. I found a 86' SVO complete rear end. I bought new calipers and rotors and brakes for it. I took my brake like off of the 4cyl. rear end so that the fittings matched up to the brake line on the car. Im having a problem with bleeding the rear brakes though. Ive bled them and seem to be getting only fluid and not air but when i push the brake pedal under normal pressure the pedal will go to the floor, but if i pump the pedal a couple times it stiffens up. The brakes are applying to the fronts and rears but the problem is with the pumping of the pedal. Im not sure if its air in the system somewhere from having all the brake lines disconnected while i upgraded the suspension and brakes or if maybe the master cylinder cant handle the new brakes or if it is worn out itself. I kept the stock master cylinder, brake booster, and proportioning valve. ive read that i should upgrade those since i did the brake upgrade but ive also read where it works fine without upgrading. this car is just a daily driver and im just struggling to figure out what could be causing the brake pedal to go to the floor but still pump up.