If the system is bled AND THE LINES are not expanding (IE thinwall aluminum tube or hoses) Once they are "full" volume in the lines should not be much of an issue. I don't know of any common passenger cars that use larger than 3/16, though.
I'm about out of suggestions, without "being there."
If the system is bled and you have a good pedal, it seems to me this just about has to be a master cylinder issue.
It IS possible to have an air problem or a volume problem whereby you are bottoming the front piston and it would SEEM like a low pressure problem, when in fact you are essentially running the front section "to the end" of travel
Have you actually rigged a gauge to the system to see what you really have for pressure front/ rear?
About 50 bucks should get you a decent enough gauge and some adapter fittings, tee, etc.
