By Jim Kerr

May is Car Care month. Its time to remember all those routine maintenance items on your vehicle that need looked after. Other than oil changes, most of us wait until something is wrong before getting our vehicle serviced, but this can be false economy. One area that very few owners ever think about maintaining is brake fluid. Not just checking the level, but changing the fluid too.

Many manufacturers recommend changing brake fluid every two years. As procrastinators, we usually wait until the brakes need major work and then assume everything was changed. The fluid may have been changed at that time, but not always. It depends on the type of brake work done, and operating with old brake fluid in your vehicle can decrease brake performance and cause expensive brake parts to malfunction sooner.

There are several types of brake fluid on the market. The most common is DOT 3. Brake fluids must meet standards set by the Society of Automotive Engineers (SAE). DOT 3 (Department of Transport) fluid must have a minimum boiling point of 401 degrees F, be compatible with other materials in the brake system, have anti-corrosion properties and mix with other DOT 3 fluids without changing chemical properties.

Some European car manufacturers specify DOT 4 brake fluid. Like DOT 3, this is a glycol-based fluid, but it has a higher boiling point. DOT 4 boils at a minimum of 446 degrees F. This higher boiling point is necessary on vehicles expected to generate a lot of heat in the braking system. When brake fluid boils, vapour bubbles are created in the sealed brake systems. When you step on the brake pedal, the brake pressure compresses the bubbles, not applying the brakes with full force. Braking performance decreases and the pedal feels soft or spongy.

One of the problems with glycol-based brake fluids is that they are hygroscopic. They attract moisture out of the air into the fluid. Leave a can of brake fluid open on the bench or leave the cap off your brake master cylinder and the fluid becomes contaminated with water. Moisture can even be absorbed slowly through the seals in good braking systems. Water boils at only 212 degrees F (100C), so this significantly lowers the boiling point of the brake fluid. DOT 3 fluid with only 3% water in it boils at only 284 degrees F. This is 115 degrees lower than clean brake fluid. DOT 4 fluid boils at 311 degrees F, 135 degrees lower than clean DOT 4 fluid. You probably wouldn’t notice this in every day driving, but if you had to brake hard, then the additional heat in the brake system could boil the fluid and you find yourself with poor
performing brakes.

Water in the brake fluid also causes corrosion. Parts such as wheel cylinders and brake calipers are not too expensive to replace, but those ABS hydraulic units can easily add a thousand or more to the repair bill. Changing the fluid will not only improve braking performance but prolong the life of hydraulic brake components too.

Silicone brake fluid, usually referred to as DOT 5, doesn’t absorb moisture from the air, and it won’t attack painted surfaces like glycol-based fluids if you spill it on the fender or grille, but it is not without faults. The viscosity of silicone brake fluid is different than DOT 3 and DOT 4 fluids, so most manufacturers don’t recommend it for use in systems with ABS brakes. The biggest disadvantage of silicone fluid has to be that it is more difficult to seal. DOT 5 fluids will seep past seals and gaskets that would normally contain DOT 3 and DOT 4 fluids.

One of the big advantages of DOT 5 silicone fluid is it has a higher boiling point than DOT 4, boiling at 500 degrees F. Silicone brake fluid and Glycol-based fluids should not be mixed. The complete system needs to be disassembled and cleaned before filling it with silicone fluid.

What’s new in brake fluids? Non-silicone based synthetic brake fluids. Sometimes referred to as DOT 5.1, it is compatible with DOT 3 and DOT 4 brake fluids. This fluid is advertised as “synthetic” but has much the same characteristics as DOT 3 and DOT 4 fluids. This biggest advantage is its higher boiling point of over 500 degrees F, similar to silicone-based fluids. As long as the container says it is compatible with DOT 3 or DOT 4 fluids, you can mix it or top up the brake system with this synthetic fluid. Better yet, take the vehicle in for service and have the old fluid bled out and new fluid installed in your brake system. The brakes will perform better, and you will have delayed the onset of more costly brake repairs.

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