Adjustable brake prop valve worse than OEM?
 

Based on some info I found, the stock prop valve that is bolted to the ABS unit on the FD has a built in metering valve that bypasses the rear prop valve until the pressure has reached over ~400psi. This means the front to rear pressure is 1:1 until ~400psi is reached at which point the fluid for the rear brakes gets sent through the prop valve which reduces pressure to 60-70% of the base pressure. I believe the way an aftermarket inline prop valve works is by simply reducing pressure to the rear wheels depending on where the spring tension is set.
I tested the pressure at the front and rear calipers with a fully stock brake system and I got 1300psi front and 900psi rear with the engine running (vacuum assist). That's a little over 70% being sent to the rear but its hard to get accurate numbers since it depends how card you press the pedal. Unfortunately I did not test the pressures at light pedal force to see if the pressure is 1:1 up to a certain point. My concern is that if you were to set the same 70% pressure reduction on an aftermarket inline prop valve, the pressure will be 1:1 all the way until 900psi is reached at the rear calipers since the valve will only close at a specific point unlike the stock system that utilizes a metering valve in conjunction with a prop valve.

Does anyone know how this would impact the cars handling at medium braking force on the track or autocross?

My theory on "why proportioning valve" is that it's a relic of the past-century 3-channel abs. The harder you decelerate, the more weight is transferred to the front wheel. When one axle has more normal force, it naturally has more grip & will require more brake force to lock (conversely, the axle with less weight needs less pressure to avoid lock up). Without the direct control of individual rear wheels (LF+RF+Rear = 3 channel), it was likely deemed a safe design to have brake bias grow forward as deceleration increases.

Will it feel weird at the track? I couldn't feel the "knee" point. Is it a necessary design? No, because a modern ABS should be able to handle each wheel's lockup individually.

Food for thoughts!

Some more information:
https://flyinmiata.com/pages/tech-st...tioning-valves
https://www.rx7club.com/3rd-generati...valve-1118774/

I came to the same conclusion after looking into it. The reason why I was asking is because I was deleting the ABS and was debating on keeping the prop valve. I ended up deleting everything and running a regular adjustable bias valve for the rear brake line, hopefully it works well without locking up the rears but I am still nervous about it. I know everybody that deletes the ABS does it this way but I am still unsure since the way bias valves work is they sent equal pressure until the set pressure is met. For example, if the prop valve is set to 800psi, the front and rear brakes will see the same line pressure as you increase brake pressure until 800psi, then the front pressure will continue increasing while the rear stays at 800psi. Most people don't know this and think an adjustable bias valve just reduces the rear pressure by a certain percentage regardless of the pedal pressure.

I don't think you missed anything! If the rear locks, just dial the bias more forward. The goldilock setup is ofc 4 wheel locking at the same time. Just keep it in mind that it's very environment dependent, lower mu = lower peak decel --> more rear bias

I think maybe you are misunderstanding how prop valves work. What you are describing sounds like the knee point.
An adjustable prop valve just replaces the stock proportioning valve and makes the knee point adjustable.
This effectively changes the amount of rear bias, since you will have a different amount of rear pressure at a given pedal pressure.
Unless you've changed the brakes (or grip/downforce) significantly, the stock one will work fine on the track in most situations.
That said, adjustment will let you optimize your braking for the car and the conditions.

You want the pressure to be equal at lower pressures so the pads and tires wear more evenly.