Where DOT 5 fluid works best is in vehicles that sit for long periods of time (antique and classic vehicles that are stored during winter months or in a museum), or in vehicles that are operated in extremely wet environments (including off-road).

DOT 5 also cannot disperse moisture that enters the system throughout the fluid like DOT 3 and DOT 4 fluids. Any water that does get inside tends to "slug" and settle to the lowest point in the system

silicone fluid contains more dissolved air and aerates more easily when pumped rapidly through small orifices that can increase pedal travel and reduce pedal firmness (a spongy pedal). Because of this, most vehicle manufacturers warn against using DOT 5 brake fluid in any vehicle equipped with ABS.

Auto Tech: Brake fluid is frequently neglected

By Jim Kerr

Know what the most important feature on your vehicle is? You probably guessed right - it’s the brakes. Know what the most neglected part of your vehicle is? It’s brake fluid. Yet, many auto manufacturers don’t have any recommendations for changing brake fluid. Check the level and if it is up, then all is okay. New studies show however, that changing your brake fluid can protect your braking system from expensive corrosion.

In Canada, brake fluid specifications are regulated by Transport Canada. Rather than specify exactly what brake fluid is, they specify all the criteria it must meet such as ability to flow in cold weather, boil at high temperatures, and be compatible with different types of materials in the brake system. In the U.S., the Department of Transport (DOT) also has similar specifications and they label the fluids according to how they meet the different classes. We also use these DOT classifications.

Most auto manufacturers use DOT 3 type brake fluid. A few use DOT 4 type fluid, which has a higher boiling point. These fluids are glycol-based and as such are hygroscopic. That means they will absorb water. Leave a container open and it will absorb water from the air. In the brake system, it will slowly (over years) absorb water through the rubber hoses and parts.

The other type of brake fluid is silicone-based. It is classed as DOT 5.0 or 5.1 type. This fluid has a blue dye added so it is visibly different from other brake fluids. Advantages of silicone brake fluid are that it is not hygroscopic and it will not damage painted parts, but it does pass through smaller pores so it does not seal as well. Most auto manufacturers use DOT 3 fluid at the factory and the two types of fluid should never be mixed. If you want to use silicone-based brake fluid, the entire system should be cleaned of the glycol-based fluid.

So when should you change or flush brake fluid? Research done in the U.S. by the Maintenance Services Task Force of the AMRA (Automotive Maintenance and Repair Association) has found that common misconceptions exist around brake fluid. The most common is that water in the brake fluid causes corrosion and sludge deposits in the brake system. Another common misconception is that dark-coloured brake fluid needs to be changed and light amber brake fluid is still good. The AMRA represents the auto service industry, equipment manufacturers, parts manufacturers, education and the scientific community and their finding are surprising.

First, the colour of the fluid is no indicator of fluid quality. Some brake fluids naturally change colour from light to dark amber as they come into contact with brake system rubber components. In their testing, they found vehicles with dark brake fluid that was still good and vehicles with light amber fluid that was poor.

Because brake fluid is hygroscopic, water in the brake fluid lowers the fluid boiling point so it may boil and not work effectively. Water will also cause corrosion problems, but there are no accurate service tests for water content of brake fluids. However, the Task Force research found that newer vehicles have very little water absorbed into the brake system. See-through plastic fluid reservoirs limit opening the brake system to the air and improved rubber materials seal moisture out better. So if it isn’t water that causes problems, what is it? It’s copper.

Corrosion inhibitors in the brake fluid reduce the breakdown of materials in the brake system. As these inhibitors wear out due to age, water content and high braking system heat, copper in the brake line material starts to corrode. When enough copper ions are present in the brake fluid, they act as oxidizers and start to corrode other parts such as ABS valves and master cylinders. The amount of dissolved copper in the brake fluid is a good indicator of brake fluid quality and the Task Force has recommended a limit of 200 ppm copper before changing fluid.

There are ways of testing for copper content. The patented FASCAR technology uses paper strips that change colour in relation to the amount of copper. If your service shop doesn’t have or use these strips, then they don’t really know if your brake fluid quality is good or not. Neither do you, so to be on the safe side perhaps it is time to flush the brake system with clean brake fluid. It could save you from expensive corrosion-related repairs and ensure your brake system stops you every time you step on the pedal.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automobile Journalists Association of Canada (AJAC).

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DOT 5 also cannot disperse moisture that enters the system throughout the fluid like DOT 3 and DOT 4 fluids. Any water that does get inside tends to "slug" and settle to the lowest point in the system