I know this is an ancient thread, but I just wanted to add what worked for me in case it helps someone else. I just ran into this issue after doing a complete brake system overhaul: Calipers, rotors, pads, complete drum hardware, drums, shoes, wheel cylinders, rear brake lines.

I bled the lines in order: pr, dr, pf, df. No bubbles in any of the lines when I was done. Brake pedal was firm and brake power was good and even. Brake warning light was on from the first time that i started the Jeep. I drove it around several miles, tried tapping the fluid reservoir, tried stepping on the brake fast and hard. Still on.

What worked for me was starting the engine, shifting to neutral, stepping slowly on the brake all the way to the floor, put it in drive, moved forward a few feet, stepped slowly on the brake. About halfway down, I heard a slight "pop" (can't tell you if it was electrical or mechanical) and the light went out. Hasn't come on again and the brakes are still working great.