If the proportioning valve is working properly, bleeding the brakes should set the dash light off from loss of pressure in the side being bled. Once the air is out (and the fluid can develop equal pressures between front and rear on the valve) the switch should self center and all is well. If the valve sticks the light on after you have all the air out, just crack one circuit or the other open, slowly depress brake pedal and when light goes out, holler at the helper to close it off. A badly stuck valve might require both circuits be done in this manner to fix. That is how I remembered this in the pre-anti lock days.
Like Johnny Popper said, don't go ape crap on things when bleeding the brakes. That's how master cylinders used to get ruined, by the piston traveling too far from an overly enthusiastic press of the pedal with no hydraulic resistance. Don't jamb it to the floor when bleeding the brakes and you should be good.
Great question though. If I'm mistaken, someone let me know.
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