I bled mine yesterday after doing some work on the modulator and it all bled up lovely with a nice firm pedal. Until I was just finishing off the last bleed nipple (nearside front caliper on the pressure system). Suddenly the pedal went 'limp' and no fluid was being pushed out.
A quick check re-bleed of the master cylinder bleed point on the modulator unit (lower forward one, the first one you bleed) released a slug of air that had found its way in there. Once clear again full system operation was restored.
I'm not saying that this is the same as your problem but it might be worth a go.
1996 2.5 DSE Auto - Bought at 101K, now on 180K - Hybrid late model ZF4HP24 Autobox - My first ever Range Rover (2nd ever Land Rover)
1971 Reliant Scimitar GTE - Jag V6 Powered Project
1979 Reliant Scimitar GTE (Omega V6 powered) - in bits!
2003 Audi TT (don't judge, it was the same price as the knackered old Corsa alternative)
PPC Magazine - now defunct