The Genuine BMW Brake Master Cylinder O-Ring - 34336799317 is a small, circular rubber ring that is used to seal the brake fluid within the brake master cylinder. It is a genuine BMW part and is designed specifically for use in BMW vehicles. This O-ring helps to prevent leaks and maintain proper pressure within the brake system, ensuring safe and reliable braking performance. It is an essential component for maintaining the overall functionality of the brake system and should be replaced if damaged or worn out.