Look up combination valves. I think the OEM ones are true combination valves. The common proportioning valves just try to limit the pressure to one circuit. A combination valve is different.
https://techtalk.mpbrakes.com/combination-valves/combination-valves
This is from an automotive engineering site. The image is the same as all the ones that describe ports to brakes. So, if you want your rear brakes to activate before the fronts, hook up the front line to the front port.
Tandem Master Cylinder
The working of tandem master cylinder.
When...
I haven't disassembled my 80 cylinder, but if the pistons are separate, then the primary (closest to the booster) builds pressure first. The secondary shouldn't move until the primary pressurizes and starts pushing the secondary piston.
Now I have concerns as well. Looking at almost every diagram of dual circuit master cylinders, the front port goes to the rear brakes. 🤔 And this shows the same thing and then it doesn't. the confusion is real. Front vs Rear ports on master cylinder? -...