I had a nightmare with exactly the same problem after changing the rear calipers ...
So first question - why are you bleeding the system - any new parts??
If not did you remember to clamp the brake bias valve BEFORE bleeding the system??
Lastly - jack the back of the car up as high as you can for a good few hours, over night is even better. The air will make it's way top the back of the system, then bleed the rears first. Remember the system(s) run diagonally so you'll have to bleed all four calipers to get good pressure from the brake pedal.
Hope this helps ...