Description: Brake fluid is a specially formulated liquid used in the brake hydraulic system. Brake fluid must meet one of three (DOT) specifications. DOT3 and DOT4 are glycol-based fluids, which absorb water. DOT5 is a silicone-based fluid and does not absorb water. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't. Most cars use DOT 3 fluid from the factory.
Purpose: Since liquids can't be compressed, brake fluid transmits force to various parts of the braking system when you step on the brake pedal. Brake fluid must also have a high boiling point because of the heat generated during braking and must not freeze during cold temperatures. DOT 3 and DOT 4 fluids also attract small amounts of water that may collect in the brake system.
Maintenance Tips/Suggestions: Check the brake fluid level in the master cylinder reservoir regularly, ideally at every oil change. Most cars use semi-transparent reservoirs that have level markings to make fluid monitoring easy. If you need to add fluid, use only the type of fluid recommended in your car's owner's manual. Do not leave the cover off the master cylinder any longer than necessary as DOT 3 and DOT 4 brake fluids attract water. Use extreme care when handling DOT 3 and DOT 4 fluid as it quickly destroys paint if spilled. On cars with disc brakes, it's normal for the fluid level to gradually drop as the brakes wear. This fluid fills up the space left by the disc brake caliper pistons as they move outward with brake wear. However, if you find that brake fluid needs to be added frequently, there may be a leak in the hydraulic system. Have the system inspected as soon as possible by a qualified service technician. Your owner's manual may specify periodic flushing and filling of the brake hydraulic system, which should not be overlooked. This is a service best left to professional technicians, as many cars with ABS have specialized brake-bleeding procedures. Braking systems with ABS can also generate extremely high hydraulic pressures, which can be dangerous. Once again, consult a professional service technician if your car needs ABS or hydraulic system service.