Single or dual diaphragm can get confusing if one does not know the size of the diaphragms. The size ( coupled with the vacuum) is what multiplies the force applied at the pedal. Dual diaphragms are typically used due to space constraints or when more pressure multiplication is needed..

I was just thinking, you can use one of your daily drivers to repeat the booster test assuming they have power brakes. As before, pump the pedal several times to empty any residual vacuum. Apply a steady firm pressure, start the car and the pedal should drop. it will give you a "feel" for what you should expect with the other system.

As far as the porp valve goes they are sometimes nothing more than a pressure differential valve that are mislabeled.

A proportioning valve will control front to rear pressures

A differential valve does not, is is only there to activate the dash light if the valve sees a drop in pressure on one side. it's other purpose is to split the front and sometimes rear lines.

Different hydraulic systems control pressures a multitude of ways, pedal ratios, boosters, MC bore diameters ( Some are stepped internally to provide different pressures to the front and rear), Piston diameters on calipers and wheel cylinders, (Again some wheel cylinders are stepped for the front and rear shoes on the same wheel as are some multi piston calipers, The surface area of the drum/rotor, the material they are made from, and finally Pad/shoe size and friction compound(s) used

Confused yet ??? Hopefully not, keep us posted