When should you change your brake fluid & does it deteriorate?

When should I change the brake fluid?

Out of all motorcar liquids, brake fluid is the most important. It transmits pressure from the brake pedal to the brakes and is analogous to brake cables on a bicycle.

Yet, modern footbrakes employ hydraulics, not cables. 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. This highlights why you should change brake fluid periodically.

What is brake fluid?

'Brake Fluid' is synthetic hydraulic oil, with the crucial property of 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 in 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 on the market, which 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 converted 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 (LHM) for their brakes. Employing mineral oil in a system that requires brake fluid (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 master cylinder reservoir, usually towards the back of the engine, close to the bulkhead/firewall. 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 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 (usually the brake pads) 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 as they age. The chief issue is water contamination that enters the system from the atmosphere. While the MOT Test includes a mandatory visual check for contamination, it 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 are 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 within the brake lines instead of applying the brakes.

When you see water boil, air bubbles form. The same situation occurs with brake fluid. As friction brakes create 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. Depending 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 around 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 brake fluid reservoir. Unfortunately, the tools needed are expensive but you could ask your garage to do the work.

You might 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 be earned. These tools tend to be inaccurate and GEM does not recommend them.

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 optimum braking performance. For more detailed information, consult our other blog on the topic: www.motoringassist.com/news/brake-fluid-when-was-yours-changed-last .

Can you drive with low brake fluid?

Never drive with a level that has fallen below the "danger" marking on the brake fluid reservoir. The brakes could be at risk of failing. Should the level fall suddenly, consult a technician immediately for advice.

Does changing brake fluid improve braking?

Yes. Apart from flushing out any contamination and air bubbles, your brakes' high-temperature performance will be enhanced significantly.

Can I top-up brake fluid myself?

Yes, brake fluid is easy to top up, which is why brake fluid is available in motor accessory stores. Remove the lid and peel off the air-tight seal. Carefully, top-up the fluid reservoir to the maximum mark and refit the brake fluid reservoir cap. Brake fluid is not only toxic but it also is an effective paint stripper. Should you spill it accidentally, flush the area with clean water but avoid getting any water into the reservoir.

Flushing the fluid through the system is a more advanced task and is best left to a technician. Do not be tempted to suck out the old fluid with a syringe and top up the reservoir.

Brake fluid FAQs

Is it normal to lose brake fluid over time?

No. Brake fluid should not be lost in the system and any leaks are highly dangerous. A slight drop in fluid level is normal to compensate for friction lining wear but this is a very different situation to fluid being expelled from the braking system.

Does unused brake fluid go bad?

When the seal is removed from the bottle, the fluid starts absorbing moisture from the atmosphere immediately. Therefore, old brake fluid should be discarded. Consult the information on the bottle label (or from the manufacturer) about shelf-life.

What happens if I never change my brake fluid?

Your braking system's performance will be affected, especially its ability to sustain prolonged braking situations. Neglected brake fluid is an indicator that the rest of the brakes might also require attention,

Does low brake fluid mean you need new brakes?

If the brake fluid drops, due to the hydraulic system compensating for wear and not a leak, a very low level might be a sign that the friction brakes might have reached their maximum wear limits.

Does brake noise indicate a problem with brake fluid?

Not usually. Noise is more likely to stem from the friction side of the braking system.

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