Driving about 45mph down the road and car in front of me suddenly stops. I immediately hit the brakes and had plenty of space to stop but truck wouldn't stop and brake pedal went all the way down to floor and I rear ended the car in front of me. It wasn't snowing, raining nor was the road wet but it was a cold day. Not sure why it didn't stop looking back on it felt like if there was air bubbles in the brake line which caused a delay in stopping, also remember feeling antilock brakes engaging and automatic emergency stop not braking for me when I was close to other vehicle.