This is a comment by Scott at SACC regarding the proportioning valve and stock brakes:
"David Nunn is absolutely correct. The question is IF you have upgraded your rear Calipers. If the answer is no then DO NOT remove the stock p-valve. If you have an upgraded rear Caliper then by all means, removed the p-valve.
Someone said you don't have a stock car so remove it. But I think you said your brakes were stock. What isn't stock is you have bigger rear tires. That doesn't help the situation, it makes it worse. You have more braking potential in the back than you did when the car was stock. So improving the fronts by removing the P-valve will also cause the back to be less effective.
We have done lots of testing to prove this theory. People still want to debate it but this issue has been tested and there is no question that a car with stock rear Calipers will have better braking performance with the valve in.
Those that claim their cars braking was better with the valve out and stock rear brakes are likely incorrect. It may feel like the fronts are stronger at the expense of the rears doing almost nothing. Measure actual braking distances and you will find a stock braking system is better balanced with the p-valve in place."
Scott