[Coda]

Well, fortunately, it's not actually an excuse, in that it doesn't excuse anything. Neither the insurance company nor the law care about your brakes failing. As the operator of the vehicle, you are responsible for making sure it is in proper working condition before driving it. If you hit someone from behind, it is almost always your fault, because you should have avoided being in the circumstances that made such a collision possible in the first place.

As a side note: It's possible that he meant that his brakes didn't work to slow him down sufficiently, not that he pushed on the pedal and nothing happened. At that point he'd be technically telling the truth, but it puts him even MORE at fault because it means he was driving too fast and following too close (and possibly didn't pump the brakes when the wheels locked).

EDIT: Now, there's a small exception in that a truly unforeseen, non-negligent mechanical failure could possibly be an excuse. But that would be a really difficult thing to have happen... If someone cut your brake lines for example and that was the first time you'd tried to slow down since you turned the car on, or if road debris flew up under your vehicle and damaged something and you had no way to avoid it... But that basically never happens.