1977 Buick Lesabre Rear Brakes not working

'77 2 door Lesabre v8 350 oldsmobile eng. Code R has only 54,000 miles on it and it also has a posi-traction rear end. I have replaced the master cylinder, both front calipers and hoses. Both rear wheel cylinders and shoes + new hardwark kit. Bleed the system 5 times, and pedal still goes almost to the floor, brake warning light is on constatly. Last week on a really rainy day stop at a stop light and both rear tires just started to spin. I read somewhere about a proportioning valve or something that needs to be reset or something before bleeding the system? Is this true and if so how? I appreciate any info you may have on this. Thanks, Ryan