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Replace your brake fluid


Replacing brake fluid can often be overlooked at regular service intervals. Unfortunately brake fluid is as vulnerable to degradation as any other lubricant or fluid in the vehicle more so when the vehicle is not being used.
Corrosion inhibitors and stabilisers in brake fluid deplete over time and need to be changed regularly.
These are ‘hygroscopic’, which means that any moisture within the braking system is absorbed throughout the entire system rather than collecting in spots. This reduces the chance of localised corrosion or boiling.
Moisture is drawn through the master cylinder cap as well as through the brake hoses.
In poorly serviced older vehicles it is not unusual to find fluid that has 7-8% water content. This moisture content slowly thickens the fluid, reduces its ability to flow and also decreases its ability to stop corrosion.

One of the most significant changes that increased moisture brings is a reduction in fluid boiling temperature. Brake fluid can commonly see 150-200'c at the brake callipers and a 2% water content can result in a 90'C drop in the boiling point. This combination of lower boiling point along with high temperatures can introduce vapour lock under adverse braking conditions, with potentially fatal results.

DOT3 specification brake fluid can reach its recommended service level of 175'C in as little as one year in service, while DOT4 specification brake fluid will reach this level after about two years.

While many manufacturers recommend a fluid change every two years, a prudent fluid change interval for these fluids would be every year for DOT3 and every two years for DOT4.
Rather than relying on time based intervals alone, it is also possible to test the boiling point of a fluid in your workshop with the correct testing equipment. This can either be a dedicated brake fluid refractometer or a tester that actually boils a test sample of the fluid to measure its boiling point.


Premature seal failure

The most common cause of premature seal failure is contamination of the fluid while servicing.
Swelling of the rubber seals occur because of the introduction of petroleum-based products such as engine oil, power steering fluid and other lubricants, even a small amount on a cleaning rag is enough to do damage. Contamination can also come from abrasive dirt or foreign material entering the reservoir when the cap is removed. The cap and surrounding area should be clean before removing the cap and make sure that the cap is clean and dry before refitting.

Storage and handling

Brake fluid should be stored in its original container in a clean and dry location away from heat. Once opened, replace the lid immediately after use, and use the entire contents as soon as possible. It is also important to tighten the lid on a 20 litre brake fluid container when not drawing out fluid. Many workshops now only buy smaller bottles of brake fluid to reduce moisture absorption in the workshop. Brake fluid is chemically similar to many paint solvents and therefore has a detrimental effect on automotive paints. If any spilled on paintwork wash it off immediately with water, do not wipe it off with a rag.