When you say a master cylinder is biased, are you saying you get different pressure out of one line VS another? Is that possible?
I don't think it is.
When a car is designed, the size of pistons front or back gives the manufacturer the calculated brake capabilities.
And that factory valve that most call a proportioning valve is not that at all. It is a shuttle valve that gives you the dual/failure mode braking if front or back leak/fail. This is not done in the master cylinder.
I've been wrong before, so I am correctable, eh?