Rear Brakes Won't Bleed?

[Kisner]

With the help of my mechanic friend, replaced my 68 coupe's rear wheel cylinders and rear parking brake cables this past weekend (both cylinders were leaking). After we put everything back together (along with all new springs), we couldn't get the brakes to bleed. Guess we tried for an hour and a half. Fluid initially dropped in master cylinder but then stabilized and didn't drop any more. Not a drop of fluid would bleed. Think we tried everything. The front brakes bleed fine. Could my lines be clogged? If so, what do I need to do - remove the lines from the master cylinder all the way back and blow them out with an air compressor? Any help would be appreciated.