it is a pressure differential switch, it is designed to sense an imbalance between the front and rear brakes, the switch needs to be centered in order for the light to go out, after you replace the master cylinder and bleed the brakes, have someone open one of the bleeders (front of you bled the back last) and very gently press the brake peddle until the light goes out, if you go to far or nothing happens go to the other end of the car (back to front not right to left) and try again
