Rear brakes lock up, front factory disc 67 Barracuda

Update... I finally got around to replacing the slave cylinders on the 69 fastback. It's got a 318 and a 7-1/4" rear-end so I used the 7/8" cylinders. I also have an adjustable proportioning valve. The brakes are now much better, meaning the rears aren't nearly as sensitive as before when I had the bigger slave cylinders but they will still lock up on hard braking. It almost seems like the fronts (Kelsey-Hayes) are not grabbing as they should. Any suggestions?