Brake fluid is a hydraulic brake system's lifeblood. It's what keeps the whole system running smoothly. DOT 3 and DOT 4 are two different types of brake fluids that are used for different purposes. DOT 3 is an old type of brake fluid used on most cars before the 1980s, when it was replaced by DOT 4 due to its poor performance in cold weather. Some people still use it as a cheap alternative to DOT 4.