FEB
What's to Stop You? The Brake Fluid 101 Guide
Choosing the right brake fluid is fundamental to your car's braking performance and is key to getting optimal braking performance from the brake components. Even the best braking components money can buy will perform poorly if the brake fluid isn’t appropriate for it.
A modified car that uses the brakes more demandingly than a standard car, may require a higher rated brake fluid than that normally specified and would also benefit from more regular changing. This is because as the brake components dissipate heat when in use, some of that heat is passed to the brake fluid. When the temperature of the fluid reaches its boiling point, it vaporises. The vapour is compressible, resulting in a depressed brake pedal compressing vapour instead of displacing fluid, the net effect being no, or very poor brakes. To avoid this, it's crucial to use an appropriate fluid, with options typically falling into three main categories: mineral, silicone, and polyglycol-based.
For classic Citroen's and Rolls Royce's, you'll likely find mineral fluid. If your car falls into this category, stick with mineral fluid and exercise extreme caution when making any changes to the braking system. Always check with the parts supplier for non-standard hydraulic parts before fitting them to ensure they work safely and satisfactorily with mineral fluid.
For all other cars either silicone, or more conventional polyglycol, fluid is used. The main difference between silicone and polyglycol fluid is that the former is not hygroscopic (it does not absorb moisture from the atmosphere). However, Silicone brake fluid isn’t suitable for most performance (e.g. motorsports) applications. This is because it becomes compressible at high temperatures and does not have a 'dry' boiling point as high as racing fluids. Also, while Silicone fluid may not be hygroscopic if any moisture or water is present or is introduced to the fluid it remains 'free' water and can produce vapour lock at temperatures as low as 100C (212F) so it still needs to be changed at service intervals.
All brake fluids are graded according to how they are affected by temperature. There are four common DOT (Department of Transportation (DOT labelling originated in the USA)) standards: DOT 3 (J1703), DOT 4, DOT 5 (silicone), and DOT 5.1. The DOT value indicates the wet and dry boiling points of the fluid. The dry boiling point is measured using fresh fluid. The wet boiling point is measured after the fluid has been exposed to a controlled humidity and has, therefore, absorbed some moisture.
The dry and wet boiling points for DOT 3, DOT 4, and DOT 5.1 are as shown in the accompanying chart.
|
DOT 3 |
DOT 4 |
DOT 5.1 |
|
|
Dry |
205C |
230C |
268C |
|
Wet |
140C |
155C |
191C |
As the chart illustrates, a higher DOT number corresponds to higher dry and wet boiling points.
The 'dry' standard is the important one because frequent changes ensure the fluid never comes close to being classed as 'wet'. Fluid changes every 12 months or less (monthly for cars used in motorsport) is recommended.
Avoid using race fluid in a road car, as it is not designed for such use, having a comparatively low wet boiling point. Also, a race fluid degrades, to the wet value, much more quickly than road use fluids do.
Although DOT 3 brake fluid is recommended for some classic cars, particularly in the USA, using it has resulted in reported problems. To address any issues that might arise with DOT 3, consider using a DOT 4 fluid, which reportedly doesn't pose any problems and comes with the added benefit of a higher DOT rating.
When in doubt, aways use the brake fluid that has been specified by the manufacturer for your particular vehicle.
