Rear brakes lock up, front factory disc 67 Barracuda

I have a 67 Barracuda notchback 6/auto with stock Kelsey-Hayes disc brakes from a 68 Valiant. I swapped the entire front brake assy., power brake and master cylinder, distribution block and brake lines. The rear is 10" drums with the lightweight rearend. The car stops ok but the rears lockup almost every time I step on the brake. The peddle feels ok (not mushy) so I think it's bled just fine. Any ideas on how to fix this?