Doesn't change the fact that many vehicles don't have that.

And they still have dual circuit brakes. Care to answer my question about your magic valve?

Go read how the meter valve works. Very low pressure it is open. It is also called a delay valve. As you apply pressure the valve closes and HOLDs off pressure to the front, while pressure builds and applies the rear brakes first, then it opens again So there is a range in which it is closed, then it fully opens again. This prevents a tail spin when braking in rain, snow. Read the whole training document. Stop picking single sentence out of context of the discussion. Plus we were talking about distribution block cutting off flow, which it does not do.

My brother went through 3 or 4 of the repops for his 70 Challenger and couldn't get any of them to stop leaking. So we found and paid up for an original set and he rebuilt them himself with the kits available from Musclecar Research.
https://www.musclecarresearch.com/

If you're determined to replace your originals don't toss them out, they're worth $200-$300 these days as cores.

For 1973 the e-body proportioning valve changed to a different design.