Rear brakes lock up, front factory disc 67 Barracuda

Update... I installed the 13/16" cylinders onto the 67 ragtop cuda which has the 8-3/4" rear-end. It worked great! On this car I have an adjustable proportioning valve and I now get front brakes locking just before the rears. I'll adjust/tweak the valve today. The other car with the 7-1/4 rear isn't bled yet but I'll get on it today.