During a hard stop, if the brakes remain engaged with hard steady pressure for too long, the brake pads can overheat and cause the moving parts of the braking system to lock in position. Brake lock up is a particular danger on wet and slippery roads. Once the braking system locks up, much of the vehicle's control and stopping ability is compromised.
Avoid brake lock up manually by releasing pressure on the brake pedal when you feel the brakes beginning to lock up. When this happens the vehicle begins to slide sideways. Pumping the brakes in an emergency stop situation can avoid the heat buildup and help maintain control while reducing the stopping distance of the vehicle.
Most newer vehicles are equipped with anti-lock braking systems. This type of brake system detects potential lock up situations and automatically pumps the brakes for the driver. On vehicles equipped with ABS systems, keep the brake pedal fully depressed until the vehicle stops; you may feel a hard pulsation in the pedal and possibly hear an associated grinding noise. ABS systems do not necessarily stop a vehicle an quicker than manually pumping the brakes, but the electronic system is much faster than most driver's reflexes.