The way I understand it the spring for the front piston in the master is weaker than the back spring so the front one will compress until the back pressure overcomes the rear spring strength and then it will compress as well. I don't think it creates a real bias. Seems more like a timing thing so that the fronts begin to work before the backs do.
I'm running a 1.25" disc/disc master cylinder. Everything I've found and everything I've seen for sale is designed to make the fronts lock up before the rears( so the rear doesn't swing around on you) but this is the situation I already have so I need something outside of the norm. A normal combo valve will make my situation worse by proporting more bias to the fronts. Just the opposite of what I need to do. What I need to do is lessen the front to rear bias. The only way I can see to make that happen is to limit the operation of the front brakes by moving the adj prop valve to the front line.
Robert
https://youtu.be/eaWBd3M9MN4