That pedestrian should not have stepped off the curb. Clearly, she doesn't see you. You hit the brakes, and she jumps back. Problem solved -- except that high squealing sound doesn't seem to be coming from the pedestrian. It's coming from your car.
There are actually several reasons that your brake pedal might suddenly go all the way to the floor when you're trying to slow or stop your car. So why would it do that? And is it really all that bad?