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
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