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When to change brake fluid

Brake fluid should be changed when the fluid boiling point falls below a certain temperature.

Apec, the braking components manufacturer and distributor, has released guidance for when brake fluid should be replaced as recommended intervals can vary tremendously depending on the manufacturer – this can be as often as every two years, to never, in some cases.

Honda states to do it every three years regardless of the vehicle’s mileage and this is also the recommended interval for most Volkswagens.

Mercedes-Benz vehicles typically call for fresh fluid every two years or 20,000 miles while for Ford, Hyundai, Toyota and other manufacturers there are no recommendations for replacing the brake fluid, only instructions to inspect it periodically.

Conventional brake fluids are hygroscopic; which means, they attract and absorb moisture from the atmosphere. This process will take place throughout the service life even though brake fluid is in a “sealed” brake system, as water vapour molecules are able to slowly penetrate the flexible rubber brake hoses.

Over time, this results in a decrease in the boiling point of the brake fluid and increases the risk of corrosion to metal components.

As the proportion of moisture in the brake fluid increases, the boiling point will eventually reach a critical level where it is dangerous. The resulting friction generates significant quantities of heat. While much of this heat is intentionally dissipated to the atmosphere, some of it will inevitably be transferred to the calipers and wheel cylinders which contain brake fluid.

This will cause the fluid to heat up. If the temperature approaches the boiling point of the brake fluid, bubbles of vapour will form in the brake system. The next time the brake pedal is pressed, its travel may all be used in compressing the vapour, resulting in a complete loss of braking. The lower the boiling point, the greater the danger of resulting ‘vapour-lock’ occurring.

Apec recommends that brake fluid should be replaced every two years or when the boiling point drops below 200°C. If the boiling point reads below 180°C, the brake fluid should be replaced immediately.

At Worcester’s Mobile Mechanic we have a brake fluid boiling point tester. Ask your garage if they have one!

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