Rear brakes locking up

[Pro69]

Hello everyone this the first time I've posted something so here it goes.
I have a 69 SWB with a 350/700r4 combo. I recently installed a 5 lug rear out a 71 chevy and the rear brakes lock up even after bleeding them several times. The front has disc brakes and a new prop. valve, master cylinder and brake booster. I installed all new parts on the rear axle prior to installing it so I dont think it has anything to do with that. I also read on this forum about the rear brake line not being any good and that causing them to lockup. Mine is in good shape also the rear e-brake is not connected. So i'm kinda running out of options. Any help would be greatly appreicated. Tks.