The importance of brake fluid

When should I change the brake fluid?

Out of all motorcar fluids, brake fluid is the most important. This is why it is included in the MOT Test.

As modern car brakes employ an hydraulic braking system, the fluid transmits pressure from the foot pedal to the friction brakes. The fluid is controlled also by the ESP (Electric Skid Protection) and ABS (Anti-lock braking systems). Therefore, without brake fluid in good condition, none of these vital safety systems works effectively.

What is brake fluid?

'Brake Fluid' is a synthetic hydraulic oil that possesses a high boiling point.

Most brake fluids are based on chemistries that are classified by the North American Department of Transportation (DOT).

Most modern cars specify DOT 3, DOT 4, or DOT 5.1 fluids. The main difference is the boiling points but most cars will accept any of these DOT-rated fluids. However, we are hearing of low-viscosity DOT 4 and 5.1 brake fluids that optimise ESP and ABS operation, especially in very cold conditions.

However, DOT 5 fluid is very different, being based on silicone chemistry. Older cars can be concerted to take DOT 5 but the system tends to need flushing and all rubber seals replaced.

Some older cars, mainly from Citroën and Rolls-Royce, use mineral fluid for their brakes (LHM). Employing a mineral oil in a system that requires brake fluids (and vice versa) will destroy the rubber seals within the system and promote complete braking system failure.

Where is brake fluid located?

Brake fluid is stored 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 top-ups. 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 wait and rely on a dashboard warning light to inform you that the level has dropped to a dangerous level.

How often does 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 a potentially lethal leak.

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

No. Conventional ether-based brake fluids (i.e. DOT 3, 4 and 5.1) deteriorate over time. The chief cause is water contamination that enters the system from the atmosphere. While the MOT Test includes a mandatory visual check for contamination, this will not establish accurately whether, or not, the deteriorated brake fluid poses a safety risk.

Consider also that the hydraulic system is only one part of the braking system. This highlights the importance of regular servicing, where the wheels are removed and the brakes checked closely. An MOT Test does not do this.

How does brake fuel deteriorate?

Brake fluid is incompressible; this 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 point must be preserved.

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

When brake fluid boils, the resultant bubbles create an extremely dangerous condition, called 'vapour lock'. To prevent this from happening, it is important to preserve the brake fluid's high boiling point, by flushing the system with new fluid. It is accepted, generally, that once every two years is a sensible schedule.

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 fluid reservoir. Unfortunately, the tools needed are expensive but you could ask your garage.

You could buy an inexpensive tool that passes a small electrical current through the brake fluid benefits and calculates the boiling point based on the resistance, but these tend to be inaccurate. Therefore, GEM does not recommend them.

When should I check the brake fluid?

Replace brake fluid, should its boiling point be deemed inadequate. Remember, it is good practice to flush the system with fresh fluid once every two years. For more detailed information, consult our other blog on the topic: https://www.motoringassist.com/news/brake-fluid-when-was-yours-changed-last

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