mechanic speak...
seems to me like you have quite a bad pocket of air somewhere, seen this before and it can be a bugger to fix.
have you tryed bleeding the brakes from the furthest from the master cylinder first then next furthest and so on? (furthest being the longest run of brake pipe) this may cure your problem. if not im stumped!
EDIT: you could also try opening all the bleed nipples at once and leaving it to drip through for a bit, just be sure the reservoir is full all the time
hope this helps
Cheers
Jake