71 disc drum brakes issue need help

[Critterman71]

I have a 71 Chevy Longhorn with power Disc and drums with with all original equipt. I am having trouble finding my problem. every time I stop the rear brakes lock up under normal pedal pressure. I can stop the truck if I ease down on the pedal but if I hit the pedal normally it always locks the rear. The booster has been tested as well as the master cylinder and the front calipers too and rear wheel cylinders and all lines. NEED HELP> Thanks Critterman71