A good discussion of proportioning and valves...

prop valve tech

The '60s FSMs called for the valve to output a constant 360 to 405 PSI with a constant 500 PSI input.

At some lower pressure (varies by model, or adjustment on adjustable valves) the rubber "stage" seal does not close, so it is basically straight-thru. Once the set pressure is reached, it is, effectively, a small piston pushing on a larger one, reducing pressure by the ratio of (the area of) one to the other. If it "equalizes", there's internal leakage (which may well be common, since most stops only last a few seconds and these weren't precision implements!)

Again, the bulk of the proportioning was done by the ratio of F/R mechanics and hydraulics, as it should be. On a typical '60s Mopar, the fronts do 60-65% of the work.

Rick