Back Brakes won't bleed

Start by removing the bleed screws at the rear wheel cylinders. That will give a bigger opening for rust junk to flow out. However, the clog is probably upstream since no flow from both. I only use silicone fluid (DOT 5) in all my cars since I had the same rust clog in my 82 Aries, but read up since many fear it for sometimes strange reasons. If you use anything else, bleed your brakes at least every 2 years to get new fluid. Glycol fluids have stuff to suspend the moisture, but when those get full, corrosion starts. DOT 4 or 5.1 are the best of the glycols. Don't fall for the "synthetic" marketing, all brake fluid is and always has been.