What is brake fluid?
Brake fluid is one of the most important but least considered fluids in the car.
Brake fluid transfers the effort applied to the brake pedal by the driver's foot to the components that stop the vehicle.
The brake fluid used in most cars looks like oil, but it's quite different, and completely incompatible with oil.
Brake fluid is very sensitive to contamination and something as simple as dipping an oily finger into a brake fluid reservoir will contaminate the system, potentially causing unexpected brake failure.
Over time, brake fluid absorbs moisture from the atmosphere. This reduces its boiling point and corrodes brake system components.
Heat generated by the brakes can cause water-logged fluid to boil, resulting in sudden and unexpected brake failure, which is why it must be changed at regular intervals.
Most vehicle manufacturers recommend change intervals of around two years, irrespective of kilometres travelled.
Grades of brake fluids
Brake fluids are categorised by a Department of Transportation (DOT) number which defines the fluid’s characteristics and boiling point.
- DOT 3 or 4 fluids are common.
- Always check the vehicle's handbook before buying brake fluid, as some manufacturers have specific requirements.
- In theory a DOT 4 fluid can be used in place of a DOT 3 fluid. However, in practice this isn't always the case as some brake system materials are incompatible with fluids other than those specified.
- A DOT 3 fluid should not be used where a DOT 4 fluid is specified due to their different boiling points.
- DOT 5 fluids are silicone based, don’t absorb water, don’t mix with conventional fluids and won’t damage paint. They are more compressible than conventional fluids and can produce a ‘spongy’ pedal feel. They are generally only used in high performance applications such as motorcycles and are not generally suitable for anti-lock brake systems.
- DOT 5.1, and its variants, offer many of the performance benefits of DOT 5 silicone fluids, including a high boiling point, without being a silicone fluid. They have a broader application than DOT 5 fluids.
Mineral oil
Rolls Royce and Citroen are two manufacturers that don't use conventional brake fluids in some of their models, instead specifying a mineral oil.
Mineral oil is not compatible with conventional systems and will severely damage the system if used.
Similarly, systems designed for mineral oils will be damaged by conventional brake fluids.
Storage and handling
Brake fluid is particularly damaging to paintwork so never attempt to mop up, wipe or rub spills. Spills should be immediately flushed away with plenty of water.
Due to its affinity for moisture don't use fluid that has been stored for long periods after opening. It's preferred to use fluid from a sealed, previously unopened container.
Warning signs
It's normal for brake fluid reservoirs to require an occasional top up, however, any vehicle demanding frequent refilling needs professional investigation as this indicates the development of a fluid leak.
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