Brake fluid is a special liquid used in hydraulic brake systems. It should be impervious to heat, freezing, thickening, and bubbling. There are different types of brake fluids made for different types of systems. These should never mixed. Be sure of what you use in the master cylinder reservouir, because, if the brake fluid gets contaminated, you’d have to change all of the piston seals and hoses. It should be noted that brake fluid is highly corrosive to paint, and care should be used not to get it on your car’s finish.
More Detail on Brake Fluid:
The brake system uses a glycol-based hydraulic fluid. The fluid is “hygroscopic,” which means it tends to absorb moisture over time (never leave a can of brake fluid open for this reason). Moisture lowers the boiling point of the fluid and causes internal corrosion in the brake system. That’s why the fluid should be replaced when brake repairs are made or every two years for preventive maintenance. There are several different types, based on the boiling temperature and other characteristics of the fluid. DOT 3 or DOT 4 is used in most passenger cars and light trucks. Use only the type of fluid specified by the vehicle manufacturer. Using DOT 3 in an application that calls for DOT 4 might create a safety hazard. DOT 5 brake fluid is different from DOT 3 and DOT 4 in that it is silicone-based. DOT 5 is NOT recommended for any vehicle equipped with antilock brakes – but it can provide long-lasting protection against corrosion for vehicles that are stored for long periods of time or are driven in wet environments.