I have had similar problems to what you describe (changed components and was unable to get a firm brake pedal after struggling with many methods over the course of many hours & days) - you are likely not getting the air out of the system even though it seems impossible that there is any air left in there.
The method I describe in post #20+ of this linked thread ended all of my "can't get the air out" problems -
http://forums.pelicanparts.com/porsche-911-technical-forum/856730-brake-bleeding-help-needed.html