Brake Proportioning Valve, Good bad or ugly Options

[Charles Freeborn]

So, on to the brakes in this new car. It still has the OEM proportioning valve for the rears. As I understand when the brake pressure reaches a certain limit (500 plus psi as I recall) it stops sending fluid to the rear calipers. My first question is does it stop and hold at that psi leaving the rears engaged to that level while allowing fronts to get more input, or does it drop the rears to 0?
The other consideration is to use a manual valve such as the Tilton. If going that route how does one determine settings without chancing spins or other drama?
Thanks.

[ChrisFoley]

The factory regulator is far from optimal on a race car, but you can make it work better by installing a longer allen screw in the end and turning it in about a quarter inch further than the stock screw. Better to replace it with something designed for precise tuning.
On my first build I put a knob type bias control valve in the middle of the cockpit where I could easily adjust it while driving but that required making custom brake lines for the tunnel.