sounds like a master issue if you keep getting air from the master port...we get this alot if you depress the brake pedal all the way down on a dry or on an old master while bleeding. once you mess up the inner seal its trashed.

Sounds like air...probably not what you want to hear. What did you do to bench bleed your master cylinder and how did you confirm that it was fully bleed? I normally plug both outlet ports and then stroke the piston...if it moves further than 1/8 to 3/16 inch before hydraulically locking and feels spongy, then you still have air in your MC. If you can get a firm pedal and it doesn't sink under steady pressure, then the MC is okay 99% of the time.

You didn't specify bore size, but that shouldn't keep you from being able to bleed the system. The rear C4 calipers have 45mm pistons, same as a C5/C6 setup, so while the caliper bodies may be small, they actually should work quite well assuming a Brembo 4-piston caliper with a piston area somewhere around 4 square inches.

Tobin
KORE3