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Automotive Brake Fluid Explained

Everything You Need To Know About Brake & Clutch Fluid

Although brake fluid is one of the most important components of your vehicle’s braking system, it is often overlooked and neglected. It needs to be changed periodically, yet many drivers don’t even know how to check it, let alone replace it.

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We have a variety of different brake and clutch fluids on offer. To learn more about what brake fluid is, how it works, and how to change it, read on.

What is brake fluid?

Brake fluid, also known as clutch or hydraulic fluid, is responsible for moving component parts within your braking system. When the brake pedal is pressed, the compressive effect builds pressure, forcing the fluid to press down on the rotors. This will then squeeze the brake pads in order to bring your car to a stop.

Braking is a high-stress, high-friction system. Because of this, you must be able to rely on your braking fluid operating properly at high pressures and temperatures. It’s vital to check your brake fluid regularly to ensure that your braking system will perform perfectly no matter the conditions.

Factors that can impact brake fluid performance

Moisture is one of the biggest inhibitors of brake fluid performance. When brake fluid ages, it can tend to absorb small amounts of moisture from its surroundings. If this moisture becomes too much, it can cause problems to arise when the fluid rises in temperature under braking. The moisture will then turn into vapour inside your

brake lines, leading to a soft-feeling pedal. Subsequently, this will stop your car braking as well as it should.

Because of this potential for contamination, you need to change your brake fluid either every two years, or every 24,000 miles – whichever comes first. This will help your brakes to continue working properly, and whilst it isn’t an easy task it is one you can do at home.

How to change brake fluid

Changing brake fluid is a simple, three-step process.

1. Firstly, you must open the cap and remove the old fluid from the cylinder. The space can be narrow, so we’d recommend using something like an old turkey baster to get to the fluid.
2. If you can reach, clean the reservoir out with a clean, lint-free cloth. This will avoid any moisture remaining in the cylinder.
3. Finally, pour new brake fluid into the cylinder. Fill it up to the fill line, and take care to replace the cap properly. And there you have it – your brake fluid has been changed.

Different types of brake fluid

Every kind of brake fluid is given a DOT rating, which will classify the specifications of the fluid. This is assigned by the Department of Transportation who sets the safety regulations for the acceptable performance for different brake fluids.

The DOT ratings given to brake fluids are based on the liquid’s dry and wet boiling points. The dry boiling point refers to the boiling point of the fresh fluid, and the wet boiling point means the boiling point of a fluid which as absorbed moisture. Generally, the higher the boiling point, the longer the brake fluid will last.

There are two main types of brake fluids:

• DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol compounds.
• DOT 5, which are based on Silicone.

It’s important to note that the types of fluid are not compatible with one another, and you should never mix these in a braking system.

Poly glycol brake fluids (DOT 3, 4 AND 5.1)

Glycol based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces.

DOT 5.1 has higher specification still and is for fast road and occasional track day use. It has a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically 900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4).

Silicone brake fluid (DOT 5)

Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4. Silicone fluid also has other advantages; it does not damage paintwork and it does not absorb water.

However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal.

It therefore requires special design considerations in braking systems. Furthermore, because it does not absorb water, any water remains as globules, which can pool in low spots in the system and cause corrosion. This water can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency.

Brake fluid boiling points

Listed below are the minimum dry/wet boiling point specifications for each DOT level.

Boiling Point:

• DOT 3 - 205°C (dry) / 140°C (wet)
• DOT 4 - 230°C (dry) / 155°C (wet)
• DOT 5 (silicone) - 260°C (dry) / 185°C (wet)
• DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet)
• Super Dot4 - 300°C (dry) / 195°C (wet) - Racing Brake Fluid

Things to consider

• Choose a brake fluid from a reputable brand, with a boiling point that's sufficient for the way you use your car. You can't have "too good" a brake fluid, so if in doubt, go up.
• Change brake fluid regularly, according to your vehicle manufacturer's recommendations.
• Never mix DOT 5 with the Glycol fluids: DOT 5.1, DOT 4 / super DOT 4 and DOT 3. Although this is important, in the real world this is unlikely to be an issue, as DOT 5 isn't widely available and is mainly used in military applications.
• Car and motorcycle brake fluids are often marketed separately (motorcycle products tend to come in smaller bottles), but there's no difference between a "car" brake fluid and a "motorcycle" brake fluid.

You can use our parts finder to find the right brake fluid for your car or van.