My brakes are driving me crazy

I have a 1968 fastback barracuda that we just placed a 340 into and want to start up but can't get the brakes to build up pressure after 4 different master cylinders. We pump up brakes open the bleeder valve and air comes out but then when we let off the pedal for a time the pedal continues to go all the way to the floor. I have been doing brakes for 20 years and never had this problem. I do have front disc brakes from a dodge dart same year and have been rebuilt. New rear brake drums, new flex lines , new wheel cylinders in the rear. Anyone have this issue before?