See Figure 1
If a loss of brake fluid occurs on either side of the diagonally split system when the brakes are applied, a piston mounted in the valve moves off center allowing the brakes on the non-leaking side of the split system to operate. When the piston moves off center a brake warning switch, located in the center of the valve body, will turn on a dash mounted warning light indicating brake problems.
After repairs are made on the brake system and the system is bled, the warning switch will reset itself, once you pump the brake pedal. The dash light should also turn OFF.