Not So Power Brakes, Help!

Bought a 68 Cuda with Year One Disc conversion on front wheels with booster and master cylinder. Need to put all 200#'s to stop the car. Checked brake shoes, drums and wheel cylinders in back. It has two different wheel cylinders based on bleed valve. One valve is super thin and the other is larger. What diameter each is, I don't know. Funny thing happened. I had the back end jacked up for about a week and when I finished and drove down the driveway, the brakes worked awesome! The pedal was smooth and the car stopped on a dime. I drove down the street all excited and could hardly stop the car when I came to the stop sign. Go figure! I have check the booster, does not leak, wheel cylinders in back are working, I checked them with drums off and both work. Adjusted drums by the book, no help. Have bled the system several times. Any suggestions?????:mumum: