No rear brakes

Hey all. My car is a 71 Swinger with the mighty /6. Front brakes are 73 disc rear is an 8 3/4. I got my ride out the other day and noticed the rear tires kept spinning when I hit the brakes. I tried to bleed them but I get no fluid out of them at all. Could my proportioning valve have gone bad? If so, where's the best place to get a replacement? Thanks.