At the beginning of the Automobile’s history, brakes were usually applied by means of levers, rods or cables. This made design cumbersome and in the event of a cable breaking, brakes were lost altogether. Hydraulic braking systems were first introduced in the early 1920s, not long after the introduction of four-wheel braking systems. The fluid that is now used in our braking systems has changed somewhat from the early brake fluids that may have contained mixtures of castor oil and alcohol.
There are basic requirements in order for modern brake fluid to carry out its important job. The fluid must be compatible with brake system rubber and metal components, resist deterioration by moisture or air, have suitable characteristics of viscosity and also have a high enough boiling point to resist vapour lock even under extreme braking conditions.
Most brake fluids today are made from mixtures of polyglycols, glycol ethers, borate esters and corrosion inhibitors. This allows them to meet existing requirements as well as being economical to manufacture. These fluids are miscible with any water that may be in the system and they are compatible with the rubber components that are found in brake systems.
Maintenance
Just like coolant, the corrosion inhibitors and stabilisers in brake fluid deplete over time and so must be changed regularly. They are also hygroscopic which means that any moisture within a braking system is absorbed throughout the entire system rather than collecting in spots. This reduces the chance of localised corrosion or boiling. The moisture is drawn through the master cylinder cap as well as through the brake hoses. After only one year DOT 3 fluid can contain as much as 2% water, depending on seal and hose condition. In poorly serviced older vehicles it is not unusual to find fluid that has 7 or 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 decrease in the fluid boiling temperature. 2% water content can result in a 90° C drop in its boiling point. Brake fluid can commonly see 150 – 200° C at the brake callipers. This combination of lower boiling point along with high temperatures can introduce vapour lock under adverse braking conditions, with potentially fatal results.
Premature seal failure
The most common cause of premature seal failure is contamination of the fluid when servicing. Swelling of the rubber seals occur because of the introduction of petroleum-based products such as engine oil, power steering fluid and other oils. Even a small amount on a cleaning rag is enough to do major damage. Cleaning solvents can also affect the rubber in the seals and so should be kept away from the fluid areas of the braking system. Contamination can also come from abrasive dirt or foreign material entering the reservoir when the cap is removed. Always ensure that the cap and surrounding area is clean before removing the cap and make sure that the cap is clean and dry before refitting.
Which fluid should I use?
Vehicle manufacturers all have their own fluid standards that vary from model to model. It is important to check these requirements before adding any fluid, especially if done so during the warranty period.
However, the general rule of thumb for choosing a fluid is to use the highest-grade fluid that the budget will allow. DOT 4 fluid can usually be used where DOT 3 is specified, unless otherwise recommended by the vehicle manufacturer. You cannot however use DOT 3 fluid where DOT 4 is specified. By following these guidelines, it will ensure that the boiling point is always well above the minimum requirements of the vehicle. It also pays to buy a reputable brand to ensure consistent quality of the fluid.
Storage and Handling
Brake fluid should be stored in its original container in a clean and dry location away from heat. Once opened, replace lids immediately after use, and use entire contents as soon as possible. Sealed containers normally have a shelf life of two years.
Brake fluid is chemically similar to many paint solvents and therefore has a detrimental effect on automotive paints. If any is spilled on paintwork, wash it off immediately with water, do not wipe it off with a rag.
Increasing complexity in brake systems and brake fluid has meant that we have to be evermore vigilant and careful when servicing and maintaining our braking systems.