Brake Fluid FAQs

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11 Feb 2025

Category:

What’s so special about brake fluid?

Brake fluid is a high-purity, high-performance liquid that transfers force into pressure, making braking possible. It has the following characteristics:

• Incompressible, providing efficient transmission of pressure to the brake with a good pedal ‘feel.’
• High boiling point, resisting the high temperatures generated by braking. (See ‘Why is wet brake fluid dangerous?’)
• Compatible with all parts of the braking system, including plastic, rubber, metal, or composite materials.
• Remains fluid at low temperatures, flowing freely in weather as low as -50°C.

Brake fluid is a safety-critical product. It may be adversely affected by small quantities of any contaminant.

What are the 3 different types of brake fluid?

1. Conventional (universal) fluids, based on glycol ethers and designated DOT 3, DOT 4, or DOT 5.1.
2. Silicone-based fluids. These can be used in normal braking systems, but shouldn’t be mixed with conventional grades. Used by some classic cars and military vehicles.
3. Mineral oil (LHM)-based fluids. These are for special braking systems, and are completely incompatible with conventional fluids. Used in some older Citroën and Rolls-Royce vehicles.

There are many sub-grades within conventional brake fluid. For example, the minimum dry boiling point for the DOT 4 standard is 230°C. NAPA DOT 4 Brake Fluid has a dry boiling point of 263°C.

Why do I need to change the brake fluid?

Conventional brake fluids are hygroscopic, meaning they absorb moisture from the atmosphere. This takes place throughout the service life even though it exists in a ‘sealed’ brake system, because water vapour molecules slowly penetrate the flexible rubber brake hoses. Fluid that has absorbed enough moisture is called ‘wet’ brake fluid.

Over time, this lowers the boiling point of the fluid and increases the risk of corrosion to metal components. The boiling point will eventually reach a critically dangerous level that could lead to vapour lock and brake failure. All European vehicle manufacturers recommend a periodical complete fluid change.

Why is ‘wet’ brake fluid dangerous?

The braking system in a vehicle works by clamping a brake pad or shoe against a moving disc or drum. This friction generates intense quantities of heat. While much of this heat intentionally dissipates into the atmosphere, some will transfer to the calipers and wheel cylinders, which contain brake fluid. If the temperature approaches the boiling point of the fluid, vapour (gas) forms in the brake system.

Unlike a liquid, gas is compressible. This means that the next time the brake pedal is used, its travel could be used to compress the vapour instead of the brake pads, leading to a complete loss of braking. This extremely dangerous phenomenon is called ‘vapour-lock’ — or sometimes ‘brake fade.’ As the boiling point decreases, the likelihood of experiencing vapour-lock increases. This can be avoided with regular brake fluid changes.

What does ‘wet boiling point’ mean?

The ‘wet boiling point’ is a measure of how quickly the boiling point of a brake fluid will decline in service. It used to be a rough approximation of the boiling point after two years, but improved braking hardware has lengthened this. The fluid is exposed to a humid atmosphere for a specified period, after which the boiling point is measured.

The minimum wet boiling points specified by the DOT standards are:

• DOT 3 — 140°C.
• DOT 4 — 155°C.
• DOT 5.1 — 180°C.

How often should brake fluid be changed?

Brake fluid deteriorates over time, but isn’t greatly affected by miles travelled. Follow the vehicle manufacturer’s advice on frequency of fluid changes. If no recommendation has been given, the brake fluid should be replaced every 18 months to maintain optimum safety. We also advise that a boiling point test is carried out at every service or brake change to identify the current wet boiling point of the fluid.

What’s the difference between DOT 3, DOT 4, DOT 5.1, and ESP?

DOT 3, the original grade, was the standard brake fluid for many years — and still is in some countries, particularly North America. However, it often suffers from steep boiling point decline. DOT 4 fluids were developed to improve upon this. They contain polyglycol ether esters, which chemically bind the water and significantly slow the decline in boiling point.

DOT 5.1 fluids are a development of DOT 4, boasting not only higher and better-retained boiling points but also a vastly improved viscosity (thickness) at low temperatures.

To meet the modern needs of ESP (Electronic Stability Program) and ABS (Anti-Lock Braking) systems, a new fluid grade was recently introduced: ESP. This grade doesn’t currently have a DOT designation, but the International Standards Organisation (ISO) has designated it as Grade 6.

Is DOT 5 the same as DOT 5.1?

No, DOT 5 is silicone-based while DOT 5.1 is glycol ether-based. Silicone brake fluids are much less likely to pick up water, and the DOT 5 standard was originally written with this in mind. However, conventional fluids were developed to meet the same standard soon afterwards — these were designated DOT 5.1.

Are silicone fluids better than conventional DOT 3, 4, or 5.1?

Only if you’re a classic car driver. Silicones have some major advantages — like their resistance to moisture — but they have some major disadvantages that often outweigh the benefits. Conventional fluids are better suited for most purposes.

Is brake fluid an oil?

No, conventional DOT grade brake fluids are synthetic. They’re incompatible with mineral oils and should never be mixed with them. Brake fluids were made from castor oil (a vegetable oil, not a mineral oil) many years ago, but this ended in the 1960s.

What is brake fluid made of?

Brake fluid, dependent on grade, is made up of polyglycols, polyglycol ethers, polyglycol ether esters, and various additives. These additives provide corrosion inhibition, pH modulation, lubricity, and rubber-swell modification.

Why do some cars need ESP brake fluid (ISO 4925 – class 6)?

ESP brake fluid has a low viscosity, meaning it’s less than viscous than normal, allowing automatic systems such as ABS and ESP to work quickly at low temperatures. These automatic systems will still work with conventional grade fluids, but could experience some loss of braking performance in extreme cold climates.

Can brake fluids be mixed?

Conventional fluids conforming to DOT 3, DOT 4, and DOT 5.1 can be mixed safely, although it should be noted that the performance of the higher-grade products will be diluted. Silicone (DOT 5) and mineral-based brake fluids should never be mixed with other types.

I have an old bottle of brake fluid. Is it still okay to use?

If the bottle has been opened, it may be okay if you know it’s under 3 months old and the bottle was tightly resealed, but generally it’s best to dispose of it and buy new. A sealed, unopened bottle that’s less than 18 months old and stored in a cool, dry place is likely to be safe to use. The age of a bottle can be determined from the batch number. You can confirm the viability of the product by taking a sample and conducting a boiling point test.

What is the significance of the batch code printed on the bottle?

It may be hard to see on dark bottles, but the batch number is typically printed on the neck of the bottle, and it consists of 2 parts. The first 4 digits are a date code denoting when the bottle was filled. The next 5 digits indicate the batch of brake fluid used. The time of day that the bottle was filled will also be present.

The date code has this meaning:

• The first character is a year code; every letter is used. Q = 2010, R = 2011, S = 2012, etc.
• The next two characters are the week number of the year.
• The fourth character is the day of the week. A = Monday, B = Tuesday, etc.

Note: other manufacturers will have their own system of date coding and batch identification.

What colour should brake fluid be?

Brake fluid colour is mandated by Federal Motor Vehicle Safety Standards Committee under their specification FMVSS 571.116.

• Conventional (DOT 3, 4, and 5.1) should be colourless to amber.
• Silicone-based (DOT 5) should be purple.
• Mineral-based (LHM) should be green.

Historically, brake fluids have been a variety of colours. Blue and red were popular in the past, and still are in some countries. However, other automotive fluids often use these colours — antifreeze and screen wash are often blue, while transmission fluids are often red. It was easy to use the wrong fluid accidentally. The FMVSS intervened and specified the above colour system in 1972 to avoid this confusion.

Conventional (DOT 3, 4, and 5.1) brake fluids may vary in colour from virtually water-white to a deep amber colour. The colour has no effect on the performance; it simply helps to prevent the use of an incorrect product.

The fluid is lighter/darker than a brand I used previously. Why?

As noted above (‘What colour should brake fluid be?’), the colour of conventional brake fluids can range from colourless to amber. Where the colour falls on this spectrum will depend on the base ingredients and additives, but most manufacturers prefer to use a colour somewhere between the two extremes.

Ideally, the fluid should have enough colour to prevent an insipid appearance and allow the fluid level to be clearly seen in the reservoir, while being light enough not to be mistaken for waste brake fluid or other red automotive fluids.

It’s also normal and expected for brake fluid to darken in storage over time; the speed of this process depends on the storage conditions and formulation of the fluid. This is not cause for alarm. If there’s any uncertainty about the age of the fluid, this can be established from the batch code. See ‘What is the significance of the batch code printed on the bottle?’

I tested some new fluid with a brake fluid tester, and it failed. Why?

It’s almost certain that you were using a conductivity test pen. While these are affordable, they can often give misleading and dangerous results, because the conductivity of new brake fluids varies significantly depending on manufacturer, DOT grade, and formulation. These testers should not be used. Instead, use a boiling point tester, which will establish the boiling point by heating a sample of the fluid. You will likely get a very different and far more accurate result. See our ‘Brake Fluid Testing’ article to learn more.

What brake fluid should I use?

The vehicle manufacturer will usually specify the minimum required performance level with reference to an internationally recognised specification, such as the DOT range. If no recommendation has been provided, a high-quality DOT 4 brake fluid from a trusted brand is suitable for almost all vehicles, except those requiring mineral-based fluid (see ‘What are the 3 different types of brake fluid?’). Most vehicle manufacturers recommend DOT 4.

What if I need greater performance?

Notably arduous driving conditions such as mountainous areas or towing a caravan/unbraked trailer can result in especially high brake fluid temperatures. Before getting started on such a journey, the whole brake system should be checked and the fluid should be changed. It is recommended that a higher grade of fluid should be used, such as DOT 5.1, for maximum safety.

The vehicle says to use LHM. Is that the same as conventional brake fluid?

No, LMH and DOT-type fluids are very different and should never be mixed. If the wrong fluid is used, it’s likely to result in total braking failure.

What does LHM stand for?

LHM stands for Liquide Hydraulique Minérale, which is French for Mineral Hydraulic Fluid. It is used in some older Citroën and Rolls-Royce vehicles.

The vehicle handbook says to use synthetic brake fluid.

DOT 3, DOT 4, and DOT 5.1 are all synthetic.

The vehicle handbook says to use fluid meeting SAE J1703. What is this?

SAE J1703 is an internationally recognised standard issued by the Society of Automotive Engineers (SAE) in the USA. It is technically equivalent to DOT 3. Likewise, the standard SAE J1704 is technically equivalent to DOT 4.

The vehicle handbook says to use a DOT 3 brake fluid. Can I use a higher specification such as DOT 4?

DOT 4 fluids from reputable brands typically meet DOT 3 standards and are compatible with DOT 3 systems, so the answer isn’t strictly ‘no.’ However, we recommend that customers follow the vehicle manufacturer’s advice.

It should be noted that lower-performing DOT specifications should never replace higher ones. For example, DOT 3 should not be used in place of DOT 4.

I have been asked about/told to use vegetable-based brake fluid. Is this fluid vegetable-based?

Development of castor oil-based brake fluids ceased in the 1960s, but some people still colloquially refer to modern DOT 3 or DOT 4 fluids as being ‘vegetable-based,’ even though they aren’t. Usually, this means the person is looking for a synthetic fluid rather than a petroleum-based product like LHM.

I have heard about ‘Super DOT 4’ brake fluid. What is this?

There is no official specification or definition of ‘Super DOT 4.’ Typically, it means that it’s a DOT 4 fluid that meets the DOT 5.1 criteria for dry and wet boiling points, but not the low temperature viscosity. Because this isn’t a regulated standard, anyone can legally call their DOT 4 ‘super’ regardless of performance — and they do. When someone uses this term, it’s best to confirm the actual performance benefits of the product.

What is the shelf life of brake fluid?

The hygroscopic nature of brake fluid means it does absorb moisture from the atmosphere to a small extent even when packaged in closed bottles. As this process occurs, the boiling point declines. The speed and magnitude of the decline depends on the storage conditions, the thickness of the bottle walls, the type of closure used, the container size and material, and the product grade.

Fluids stored in a moderate climate will have a shelf life of approximately 18-24 months from date of production. In tropical conditions, the shelf life would be 12 months.  However, brake fluid containers that have been kept in stock for a long period of time could deteriorate aesthetically, such as gathering dust or fading labels. This imposes a practical shelf life in terms of sellability.

Brake fluid stored in a closed container doesn’t necessarily become unsafe after the shelf-life expiry date. It may theoretically be safe to use fluid that’s over 10 years old — but after its shelf life has passed, it can’t be guaranteed to meet the stated specification. For example, an ‘expired’ DOT 4 bottle may not meet a minimum wet boiling point of 155°C.

Under what conditions should brake fluid be stored?

Brake fluid packages should be stored under cover in a dry and mostly dust-free environment, out of direct sunlight at a temperature of 15-30°C, to preserve the shelf life. Where possible, avoid significant changes in humidity or temperature.

How often should I check the level in the reservoir?

It’s usually recommended that all critical fluid levels should be checked weekly, but the vehicle handbook may have different advice. In the absence of system faults, most modern vehicles don’t normally need a brake fluid top-up between services.

What will happen if the level drops?

Low brake fluid levels in the reservoir carry a risk of introducing air bubbles into the braking system, which could lead to brake failure. See ‘Why is wet brake fluid dangerous?’ for more information.

How much fluid should I buy to do a complete flush of my braking system?

The exact amount needed will vary with each vehicle, but you’ll likely need 1 litre to flush and refill the system.

I topped up the brake system with LHM (or another petroleum-based product) by mistake. What do I do now?

LHM will damage the rubber components in the system, making them distort and swell until they need to be replaced. You should completely strip your entire system, replace any rubber components, and clean each metal part thoroughly with brake cleaner. Ensure each part is entirely dry before reassembling and bleeding through the correct product. The key thing is to remove the LHM as quickly as possible.

I topped up the brake system with power steering fluid. It’s basically the same, isn’t it?

No, not at all. You will need to strip the system in the same way as for LHM (see above).

How should you clean up spilled brake fluid?

For a small amount, soak up any spillage using rags or another suitable absorbent, then clean the affected area with a generous amount of water, as brake fluid is water-soluble. Larger spills should be confined using sand or earth, then pumped into a salvage container. If spilled on paintwork, flush immediately with water. Brake fluid strips paint.

Is brake fluid environmentally hazardous?

No. Conventional DOT 3, 4, or 5.1 fluids are water-soluble, biodegradable, and of low environmental toxicity. However, it’s still important that waste brake fluid is taken to a local recycling centre and never disposed of down the drain.

Can I tip waste brake fluid into the waste oil drum?

No, this is irresponsible. It’s likely that your waste collector will not accept the waste oil at all. Instead, collect waste brake fluid on its own in a clean bottle or drum and get it collected separately. Waste brake fluid can only be recycled if it’s kept separate. Domestic consumers may be willing to take waste brake fluids to a household waste site provided by your local council.

Why must waste brake fluid be kept separate from other waste?

There are 3 critical reasons.

1. By law, waste brake fluid and waste oil are both classified as hazardous waste, and the mixing of hazardous waste is strictly prohibited. Mixing them could, in theory, lead to prosecution.
2. Brake fluid does not mix with mineral oil. When these wastes are mixed, the brake fluid separates in the water phase (which is almost always present), and must be removed and disposed of at an additional cost.
3. Once the 1975 Waste Oil Directive is fully implemented in the future, waste oil will need to be reprocessed into new oil instead of used as a fuel. The government won’t be able to subsidise this conversion and must instead make provisions for recycling. Again, any oil-contaminating brake fluid will need to be disposed of at an additional cost.

Brake fluid strips paint? Does that mean it is corrosive?

No. It strips paint because it’s essentially the same chemistry as paint, and it acts as a solvent.

Is brake fluid toxic or harmful in any way?

Conventional DOT 3, 4, or 5.1 fluids are of relatively low toxicity. Their main hazardous effect is irritation of the eyes, and of course, it should never be ingested (swallowed). For further details, see the appropriate Material Safety Data Sheet.

One of our staff has gotten brake fluid in their eyes. What should we do?

Flush the eye with water for 10 minutes, holding the lids apart. If any irritation persists, get medical attention.

Will brake fluid damage or stain my hands, and how should I clean them?

It won’t damage or stain your hands unless immersed for long periods. Extensive contact may cause drying of the skin, which is reversible. Clean your hands or other affected skin with soap and water. In case of contact with eyes, see above. More details are contained in the appropriate Material Safety Data Sheet.

Help, someone (usually a child) has swallowed some brake fluid. What should I do?

Get medical attention immediately. If possible, show the medical personnel the exact bottle.

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