1969 Mustang Brake problem, low brake pedal.

I have a 1969 Mustang I changed the master cylinder, all the brake lines, the brake cylinders, bled the system multiple times and my break pedal still is low. If I plug the master cylinder the brake is hard. If I hook up the brakes for the rear and keep the front plugged I have a solid brake pedal. Once I hook up the front line the pedal goes almost completely to the floor. I have all drum brakes. I adjusted them so I have about 1 full to a revolution and a half on the brake drag. They are manual brakes no booster. I tried changing the proportioning valve and that made no difference. Anyone have any ideas or suggestions.

Thank you.