[OrrieG]

The switch is in the center of the proportioning valve and sense when the valve moves to either side indicating loss of fluid in one of the circuits. Sounds like you need to bleed again. Make sure the front calipers have the bleeder on the top, they can be flipped side to side and will not bleed out with the bleeder on the bottom. When I've had mine completely apart it has taken hours and multiple tries to get all the air out. Speed bleeders are cheap and help. If the proportioning valve gets offset there is a procedure in most manuals on how to reset it. I've never done it, mine always seemed to find there way back to center on their own.