Hi all, I've noticed that both of my rear brakes have not been engaging. My rotors on the rear have developed rust on the surface, and while my fronts do this also when not in use, but it goes away after a drive, the rears stay like this. I know the rear brake calipers can move (I lifted each rear side, removed the pads and rotors, and pushed the brake pedal and the piston moved). My ABS and traction control light are both on (I get a code of "short or plus to ground - front left, to which I replaced the ABS speed sensor, but that did not help). Any suggestions? My brake pedal overall feels fine, and braking seems to be fine. I am thinking of bleeding the rear brakes to see if this will help. Any thoughts/help would be greatly appreciated. Car is a 2002 Audi A4 Quattro V-6.

Thanks.