Hy RATRODKING, when one system develops pressure and the other one does not. The proportioning valve reads that as a hole or leak in the system with little or no pressure, and the valve shuttles over to shut off flow to that system. That can make it very hard to bleed the system that had no pressure. Try putting everything back together, and then bleed the fronts, once you get most of the air out and are able to develope pressure in both systems the valve may recenter itself by constant pressure on the pedal. Some valves have a manual reset feature, a rubber cover over the plunger rod, press the rubber cover in towards the center of the valve, until the light goes out, then bleed again very slowly, or the valve will shut off flow again. If you have speed bleeders, open one of your rears while bleeding the fronts, this should stop the valve from shuting off flow to the front system while bleeding, hope that helps.
