Have you ever hit your brakes on your 330i only to find the pedal hard as a rock? It's not a fun moment--we know--but you found you could stop the car by mashing on the pedal with both feet, as hard as you could. At least we hope that's what you found. Anyway, what you experienced wasn't a brake failure, but rather a failure of the BMW Brake Booster. You've heard the term "power brakes," right? Well, your BMW brake booster is what puts the "power" in power brakes.

The brake booster is really a neat device that uses engine vacuum to help your foot generate pressure in the brake master cylinder. It has a weak spot, however: The BMW brake booster uses a large rubber diaphragm to separate engine vacuum from the air in the atmosphere, and when rubber ages, it cracks and leaks. If that happens to your BMW brake booster, you have no vacuum to assist you and you have to throw everything you've got at the brake pedal to stop your car. Don't worry too much, though: The BMW brake booster usually develops small leaks before a catastrophic failure. So just pay attention to the signals your car is giving you and you'll catch a bad brake booster long before it fails completely.