I have a 90 G60 that i have owned since 2000. I am looking to upgrade my brakes once again. I have been running a Zimmerman setup I purchased shortly after getting the car. I have 17" wheels, Koni adjustable coilovers, aluminum rear calipers, I have replaced the ABS with a vacuum booster and master cylinder, and I have lowered it as far as I feel comfortable with (rolled wheel arches and rubs under certain conditions). Recently my left rear has began to grind. my rear rotors are clean while the fronts are rusty which leads me to believe the proportioning valve is probably not operational. I have purchased new cross drilled and slotted rotors (11" rears with kit) and pads. Is there any upgrades or modifications to the proportioning valve that I can do to optimize my setup? Any other ideas without going to big $ would be welcomed as well.