This could be really simple. You have fill the reservoir __way__ up to the top before any fluid overflows into the back chamber, which feeds the rear calipers. About 90% of the volume of the reservoir is dedicated to the front brakes - only the extreme rear bit is divided off for the rear brakes. Take a strong light and make certain you have fluid in the section for the rear brakes. When bleeding it only takes 5-6 pedal strokes to empty the portion of the reservoir for the rear brakes, so it's easy to get air into the system.
Hope this is your problem, and don't ask how I learned this...