Rear brakes lockup

I have a 69 Barracuda fastback and have always had a problem with the rear brakes locking up sending the car into a spin. I updated the drum system to 72 front disc (small bolt pattern) and changed the master cylinder and proportioning valve at the same time. All parts came off a donor 72 Valiant but the master was no good so it's a rebuild. I have the cheapie stock rear end and drum brakes and all brakes are new and installed correctly. I bled the system several times. The car never stops well but is slightly better than the old drum brakes. I also kept the power booster from the drum brake system. Any ideas on how to keep em from locking up?
Author: admin