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The importance of brake fluid

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Out of all motorcar fluids, brake fluid is the most important. This is why the MOT Test includes a visual contamination check.

As modern car brakes work hydraulically, the fluid transmits pressure from the foot pedal to the brakes. Fluid is also directed by the anti-lock braking, or anti-skid hydraulics. Therefore, without brake fluid in good condition, none of these vital safety systems can work effectively.

Where is my brake fluid?

Brake fluid is retained within a translucent reservoir, usually towards the back of the engine. It has a removable top, which may be recognisable by its bright colour, to facilitate easy topping-up.

You should check the brake fluid level at least weekly. You should be able to do so by sight alone, without removing the cap. However, do not rely on a dashboard warning light to inform you that the level has dropped to a dangerous level.

How often should my brake fluid need topping up?

The brake fluid level will drop slightly as the braking system's friction parts wear naturally. A sudden drop should be investigated immediately, because it suggests strongly that there is a potentially lethal leak.

Is all I have to do is check the brake fluid level?

No. Brake fluid deteriorates over time. The chief cause is water contamination that enters the system from the atmosphere. As mentioned earlier, the MOT Test includes a mandatory visual check for contamination but this will not establish accurately whether, or not, the deteriorated brake fluid poses a safety risk.

How does brake fluid deteriorate?

Brake fluid is incompressible. This is what makes it an effective hydraulic fluid. Air bubbles, however, can be compressed. Therefore, when air bubbles enter the fluid, depressing the brake pedal will compress the air bubbles instead of applying the brakes. When you see water boil, air bubbles form. The same situation occurs with brake fluid. As friction brakes create a significant quantity of heat, much of this energy is transferred into the fluid, which is why its high boiling points must be preserved.

When brake fluid boils, the resultant bubbles create an extremely dangerous condition, called 'vapour lock'. To prevent it from happening, it is important to preserve the brake fluid's high boiling point.

Understandably, brake fluid boils at higher temperatures than water. Dependent on the precise fluid specification, the boiling points of fresh brake fluids vary between 205 - 260 degrees Celsius. Yet, when contaminated with just under 4% of water by volume, these boiling points fall considerably to 140-180 degrees Celsius.

How do I test brake fluid?

The only accurate way to assess brake fluid for moisture content is to boil a sample, collected from your car's reservoir. Unfortunately, these tools are expensive but it is a good idea to ask your garage. You could buy an inexpensive tool that passes a small electrical current through the fluid and calculates the boiling point based on the resistance, but these tend to be very inaccurate. Therefore, GEM does not recommend them.

When should I change the brake fluid?

Replace brake fluid, should its boiling point be deemed to be inadequate. Should you be unsure, fresh fluid should be flushed through the circuit once every two years. For more detailed information, consult our other blog on the topic:

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