Large bolt pattern brake HELP! HELP! HELP!

1971 Dodge Demon manual 4 wheel drum converted to disc/11" drum. retained original distribution block and using disc brakes off a 73 Dodge Dart with large bolt pattern with calipers flipped and bleeders at highst point on caliper. reman "Advance Auto parts" master cylinder that was bled as directions for quick pickup indicated. has firm feel on bench but in car with line hooked up is soft. can hear rear drums engage/disengage as brakes pumped but pedal goes to floor on first pump. will gain some pressure if continue to pump brakes. any ideas? I didnt't use a proportioning valve...is that it? any other solutions? this is my second master cylinder