I have the same problem with my 67 Coronet. Have the 10 inch rotors and an adjustable proportioning valve and the DoctorDiff smaller (15/16"?) master cylinder. I have the Raybestos semi-metallic pads and 10 x 2 1/2 inch rear brakes.

It just doesn't have any braking power. Last year my daughter spun the car out at the track because the rear brakes locked up.

I am going to replace the calipers with the larger 76 Volare calipers. Not sure what else to do, but right now the brakes stink!