Whenever I have had this problem it's typically a bad line, or a bad master cyclinder, even if it's new. I was rebuilding a Ford RS200 that had been sitting for years which has twin AP Racing masters, I replaced them with brand new AP masters and still could not get the front to bleed, ended up I just got a bad new master from AP. I would also try to reverse bleed them, if you reverse bleed them you will know 100% that you have gotten all the air out of the system.