I had problems like that when I set up my system and that was before I added the HydroBoost.
I could bleed the fronts Ok but the rears was impossible. Situation was like yours with stiff pedal and no fluid coming out in the rears.
To solve it I used a vacuumdevice that sucked out the fluid and air from the system. With the vacuumdevice it just took a few minutes to have it working and the pedal had a normal feel when it was done.
Here is one example of this tool:
http://www.northerntool.com/webapp/w...0613_200310613
And here is a DIY simple one:
Jan