what turns on the lite,,, the proportioning valve stays centered as long as the hydraulic pressure is equal on both sides,,, when its not the valve moves to block the port that has less pressure (hydraulic failure) so you then will have brakes on one axle,,, yours has moved and blocked off the rear,,, you need to reverse what cased it to move in the first place so create a leak in the front
open one of the bleeders on the front,,, then nail the brake pedal hard as you can,,,its not necessary to push the pedal to the floor,,, the impact will move the valve,,, tighten the bleeder,,, pump brakes and check lite,,, if lite is still on are comes back on you still have air in the rear,,, repeat and bleed,,, once you git the lite off re bleed the rear but only use haft strokes on the pedal and go EZ as not to cause the proportioning valve to close off again till all the air is out,,, I don't c how you could bleed it with the lite on so suspect you still have some air in the rear