About Us > Car Care Tips > The Forgotten Fluids

By Bob McGartland

The Forgotten Fluids

Fluids are the life blood of your vehicle and are critical to long life and good performance. Most drivers are familiar with checking their engine oil and coolant levels and realize these need regular service. Fewer drivers are familiar with transmission and drive train fluids and their required maintenance. Brake fluid is the “often forgotten” fluid in your vehicle and could be the “Maytag repairman” of fluids. Few people check or service their brake fluid until they experience a brake problem or failure.

Most vehicles use DOT 3 or DOT 4 (Department of Transportation Standards) brake fluids which are usually polyglycol based fluids. These fluids cause rubber parts of the brake hydraulic system to swell slightly, improving sealing and preventing leaks. Polyglycol fluids are inexpensive, readily available and have good lubricating qualities. The downside to these fluids is their high affinity for moisture. This causes a reduction in their boiling point and internal rust and corrosion problems in the brake system. This is the brake fluid used in most vehicles today and it is very important to keep it clean and moisture free. I recommend exchanging this fluid at the minimum every two years.

DOT 5 brake fluid is a silicone-based fluid which is usually purple in color. Silicone fluids have no affinity to moisture and maintain extremely high boiling points. Their drawbacks are poor sealing and lubricating qualities, much more expensive and air retention properties. Because of these problems silicone fluids are not used in most brake systems today…especially ABS brake systems. These fluids work well in special applications such as race cars that experience high temperatures or antique/show cars that sit idle for long periods of time.

Hydraulic System Mineral Oil (HSMO) brake fluid is a blend of mineral oil and special additives and is generally identified by its green color. This fluid is typically the most expensive and is only used in certain European vehicles.

Polyglycol, silicone and HSMO fluids are not compatible with each other and should never be mixed. We always use the type of fluid that the vehicle manufacturer recommends for your car. Automotive service and repair centers such as mine have equipment to test your brake fluid so we can accurately determine the moisture content and recommend when your brake hydraulic system needs to be cleaned, exchanged and new fluid installed. Even brake fluid gets tired and worn out—periodically it needs a break if you want to brake!

Happy Motoring!