proportioning valve (to be or not be)?

I bought several new adj prop valves for ~$30 on ebay. I recall one was branded "Jegs", but all look from the same Chinese factory. Ideally, the fronts should skid just before the rears, tested in a wet parking lot. That is true regardless of drum/drum, disk/drum, or disk/disk. One could design any of these to work w/o a prop valve if you select the ideal bore calipers and wheel cylinders. But, that would also require many lab tests. So, an adj valve is best. The ideal setting also varies w/ tire changes and even weight distribution. Many trucks and vans (even minivans) have a proportioning valve which varies w/ the rear load. They have a lever which moves as the rear cabin sinks under load.