I'm going to look into the master cyl. upgrade, gm made some master cyl's that had diagonal brakes, the advantage to this is that both reservoirs are the same size, the easiest way to identify a diagonal master cyl. is the partition is on an angle, that way each part fed 1 caliper and 1 drum, otherwise you will have to check your fluid level on a regular basis,
as for locking the rear brakes if you sneezed on the brake pedal, the adjustable proportioning valve would take care of that, sure it would reduce the pressure to the rear but if their were 4 calipers with a total of 16 pistons and 8 pads you wouldn't need much pressure the difference is your not working the brakes as hard, the result would be cooler running brakes, so not only would brake fade not be a problem but your pads and rotors would last forever
hell with a setup like that stopping 44's would be cake, also if you think about it Toyota had a problem with the rear brakes locking up thats why they put in the load sensing valve, that way if you were empty your rear brake action is light and if your loaded heavy you get more rear brake application
