Andrew,

I can't help with the proportioning valve question but a couple of points about using any brakes hard and warped rotors. Overheating, in and of itself, will not warp the rotors. I run my 850 on a road track and brake right to the threshold at least two times per 1.7 mile lap with moderate braking at three other corners. I've brought it back to the pits and you can't touch the wheels with your bare hands.

You can, however, warp the rotors by heating them up and then holding the brake pads on the rotor with the brake pedal at a stop light or by setting the emergency brake when you park. The rotor cools unevenly and warps.

Once warped the problem just worsens. The recommendation is to replace, rather than turn, the rotors.