Sounds like you have a faulty master cylinder...

Had a similiar experience with my beater [87 Corolla]... had replaced a faulty/leaking caliper and was bleeding the system after that [caliper replacement] task.... car simply would not bleed...

[New] fault was faulty master cylinder >> [note: a master cylinder does not have to leak externally tobe failty] >> the internal piston seal can leak > which prevents adequate pressure to be developed ...

my .02