NOTE: Always check your vehicle's owners manual (if available) or even the cap on the master cylinder reservoir before adding brake fluid. DOT 3 and 4 are very similar fluids, though one may be specifically recommended by your particular manufacturer. DOT 5 (silicone fluid) is rarely used, but it is not compatible with systems designed for DOT 3 or 4 and, if used in the wrong system, would cause damage to the seals and other rubber components.
Brake fluid is used for both the brake master cylinder and the clutch master cylinder (if your vehicle is equipped with a hydraulic clutch). Use only brake fluid rated DOT 3 or 4 or conforming to SAE Standard J1709. The rating can be found on the container. Brake fluid is typically hydroscopic, meaning it tends to absorb water from the atmosphere, both in the vehicle and on the shelf. This is a good reason to change fluid at recommended intervals, and always use fresh brake fluid. Don't buy more than you are going to use, and purchase small containers rather than large so that you can keep them sealed for future use.
WARNING
Brake fluid makes a wonderful paint remover. Be careful not to spill any on
painted surfaces.