Classic/Antique Car Repair: brakes, drum brakes, disc brakes

Question
Purchased 1966 belvedere. Had new brake booster and  dual master. 1966 came with single. Has drums on all four. Petal seemed too soft when first applied push on down and brakes too much. (put you through dash). Looked like new master cylinder was for disc      brakes so I put new one on for drums.  Same problem. Can you please help> Thanks

Answer
There is a difference between the booster for disc brakes and drum brakes. These problems happen often when modifications are made. I would like to see the original booster in the car and make sure that the bore of the master cylinder is the same size as the original.