Rear Brakes locking up

Took my 72 Demon out for a trip to the gas station and to do a little shakedown today. Brakes seemed fine at lower speeds but once i needed to slow down faster the rear brakes locked up on me pretty easily. It has drums on all four corners. I checked the parking brake (which works great by the way) to see if it was a little tight but with it out there seemed to be plenty of slack in the cable. My next step is to check the adjustment on the rear. I just wanted to get any ideas/opinions that on anything else i should check. The shoes seemed to be right, little one on the front, big on the back. Could something as simple as being adjusted out too far cause this issue?