voltage fluctuation is normal - the hard braking is not normal... lol...

after dipping, it rebounces back, right? when such a change occurs with an external event, like in this case, hard braking, it is within the range unless the dipping is into 5-9 volts or so...

how often are you hard braking? are you racing the car? because if the car is raced, there are many other components that need to be upgraded to heavier duty versions - the brakes, the brake fluid, the cooling system, tranny cooler might need to be installed, some weight needs to be shed off the car...