[Jeb_]

I recently installed F30 front brakes and 135i rear brakes on my 128i and am looking to now swap master cylinders to regain some pedal firmness and get a reasonable brake bias. I'm trying to decide between installing a master cylinder from an 135i or one from an E9x M3.

Looking around, It seems like people frequently neglect master cylinder piston sizes when doing their calculations for brake bias. This strikes me as odd since as far as I can tell, all BMW master cylinders have different diameter pistons for the front and rear brakes, meaning that with the same amount of force applied to each piston you end up with higher pressure in the line for the rear brakes. That being said I'm trying to wrap my head around how the force applied to the brake pedal is split between the pistons. Is it split equally, is it a fixed ratio based on piston sizes, does the booster equalize line pressure? I feel like I'm missing something.