97 Trooper Brake Issue

The ABS light has been on for a while, but the truck seems to brake fine. However, I have gone through 3 brake master cylinders in 2 months and from looking at the rear brakes they don't appear to be making more than 50% contact (viewing unused portion of rotor, used portion is very far from the edge). I know the rear brakes only account for 25% of braking power but I'm at a loss for why I keep plowing through brake master cylinders. Another oddity to me is the when the parking brake is on, the car will no roll forward, but if on a slight decline will roll backwards as if the rears aren't biting at all. This leaves me to presume that the rears aren't really doing anything?