Brake Fluid Explained: Types, Boiling Points & When to Change It
Posted by Matthew Marks on 1st Aug 2024
Brake fluid is one of the most critical fluids in your car, but it is also one of the most overlooked.
It is responsible for transferring force from your brake pedal to the callipers, allowing your car to slow down or stop safely. But when it fails, it does not fail gradually. It fails instantly.
We have experienced this ourselves on our AWD turbo Civic. On its first proper test drive, the brakes overheated and the fluid boiled almost instantly. The pedal went soft, braking performance dropped off, and we had to pull over and let everything cool before driving again.
A combination of poor brake fluid, undersized brakes, and excessive heat in the engine bay caused this.
That is exactly why choosing the correct brake fluid and protecting it from heat is critical.
In this guide, we will break down everything you need to know about brake fluid, how to choose the right type, and how to prevent common failures.
What is brake fluid and how does it work?
Brake fluid is a hydraulic fluid that transfers pressure from the brake pedal to the brake callipers, using force to squeeze the brake pads onto the discs. This friction between the brake pads and the discs slows down the wheels and stops the car.
The most common type of brake fluid used in cars is glycol-based, made of a mixture of glycols and other additives. This type of brake fluid is known for its high boiling point, which makes it resistant to vaporisation and boiling.
This is important because when brake fluid boils, it creates air bubbles, which can cause the brake pedal to feel spongy and reduce braking performance.
In addition to glycol-based brake fluid, there is also silicone-based brake fluid, which is used primarily in racing applications.
This type of brake fluid has a higher boiling point than some glycol-based brake fluids, making it ideal for high-performance applications.
However, it is incompatible with most rubber seals and hoses, so it should only be used in vehicles with a complete silicone-based system or aftermarket stainless steel brake lines from companies such as Hel Performance.
Types of brake fluid
As mentioned earlier, there are two main types of brake fluid: glycol-based and silicone-based. Within these two categories, there are several different types of brake fluid that are designed for different applications and vehicles.
Wet and dry brake fluid boiling points
It's important to consider both the dry and wet boiling points regarding brake fluid. The dry boiling point is the temperature at which brake fluid will begin to boil when it is fresh and unopened.
On the other hand, the wet boiling point refers to the temperature at which brake fluid will start to boil after it has absorbed 3.7% water.
This usually happens after approximately two years of use; therefore is highly recommended to replace brake fluid every 1-2 years to ensure optimum performance and safety.
| Fluid Type | Dry Boiling Point | Wet Boiling Point | Best For |
|---|---|---|---|
| DOT 3 | 205°C | 140°C | Basic road cars |
| DOT 4 | 230°C | 155°C | Most road and performance cars |
| DOT 5 | 260°C | 180°C | Silicone-only systems |
| DOT 5.1 | 260°C | 180°C | Modern ABS / low viscosity systems |
| Motul RBF 660 | 328°C | 204°C | Track, fast road, high heat builds |
DOT 3 Brake Fluid
DOT 3 brake fluid is typically formulated with a glycol-ether base. Its minimum dry boiling point is 205°C, while its wet boiling point, measured with about 4 per cent water mixed in the fluid, is 140°C.
It is suitable for use in regular vehicles that are used for typical purposes such as commuting.
DOT 4 Brake Fluid
DOT 4 brake fluid is very similar to DOT 3, but with additives that increase the minimum boiling points. DOT 4 fluids have a superior dry boiling point but require more frequent changes.
DOT 4 fluids designed for racing or performance cars can significantly exceed the minimum boiling points – they're sometimes referred to as Super DOT 4 fluids.
Regular DOT 4 is appropriate for standard vehicles with a typical dry boiling point of 230°C and wet boiling point of 155°C.
DOT 5 Brake Fluid
Dot 5 brake fluid is incompatible with any other brake fluid type. DOT 5 doesn't attract water, acts as a rust preventer, and doesn't harm paint. It's also costly, silicone-based, and is outperformed by speciality DOT 4 fluids.
Unless your car is designed to run DOT 5 or has a specific reason to run it, you can safely ignore this fluid. The general dry boiling point is around 260°C, with a wet boiling point of 180°C.
DOT 5.1 Brake Fluid
DOT 5.1 brake fluid has similar chemical properties to DOT 3 and 4, but has a lower viscosity. It has the same minimum wet and dry boiling points as DOT 4.
However, DOT 5.1 is required by some vehicles for efficient operation.
It's important to note that mixing different types of brake fluid can be dangerous and should be avoided at all costs. This can cause the brake fluid to lose effectiveness and cause brake failure.
Some brake fluids have a higher wet and dry boiling point than the standard versions, such as the Motul RBF 660 brake fluid. This fluid is DOT4, but instead of 230°C dry and 155°C wet, it can withstand 328°C dry boiling and 204°C Wet.
This specific brake fluid is much better for track-focused use, as the braking system gets much hotter, where you will likely boil the standard fluid.
Motul RBF 660 Factory Line 500ml
A strong upgrade for track cars, fast road cars and builds that have already boiled normal fluid.
From £22.99
Motul RBF 600 Factory Line 500ml
Great for performance, road and occasional track use where standard fluid is not enough.
From £19.99
Motul DOT 3 and 4 Brake Fluid 500ml
A sensible service fluid for normal brake and clutch systems.
From £8.50
Motul DOT 5.1 Brake Fluid 500ml
Useful for vehicles that require DOT 5.1 fluid and lower viscosity brake fluid behaviour.
From £14.99
Which brake fluid should you choose?
If your car is a standard road car and you only use it for normal driving, a good quality DOT 4 brake fluid is usually enough.
If you drive hard on the road, do track days, have upgraded pads, or have already felt brake fade, you should look at a higher-performance DOT 4 fluid such as Motul RBF 600 or Motul RBF 660.
If your brake pedal has gone soft during hard driving, or the car has already boiled normal fluid, we would usually move straight to Motul RBF 660. This gives you a much higher boiling point and more safety margin when the brakes get hot.
If your vehicle specifically requires DOT 5.1, follow the manufacturer's recommendation. Do not switch to DOT 5 silicone fluid unless the braking system is designed for it.
How Heat Affects Brake Fluid (And Why Most People Miss This)
Most people think brake fluid only fails because it gets old. In reality, heat is the biggest factor.
In many performance builds, especially K-swapped cars, the exhaust manifold sits very close to the brake lines and the proportioning valve. This exposes the fluid to constant radiant heat.
Without protection, this heat transfers into the fluid, lowering its effective boiling point and increasing the chance of brake fade.
This is why upgrading fluid alone is not always enough. You also need to manage heat properly.
A good example is a K-swapped EK Civic. The exhaust manifold sits at the back of the engine bay, close to the brake lines and proportioning valve.
That means the brake system is not only dealing with heat from the brakes themselves, but also with radiant heat from the exhaust side of the engine.
In this situation, simply fitting a better fluid can help, but it does not fix the source of the heat. The better solution is to combine higher boiling point brake fluid with proper heat protection around the manifold, brake lines and nearby components.
For this type of setup, we would usually look at an exhaust wrap on the manifold and a silicone heat sleeve on the brake lines. This helps reduce the heat reaching the fluid before it becomes a problem.
Exoracing Silicone Fibreglass High Temperature Heat Sleeve 0.5m
Ideal for brake lines, clutch lines, fuel lines and wiring close to manifolds or downpipes.
From £14.99
Exoracing Gold and Silver Heat Reflective Tape
Reflects radiant heat away from nearby panels, lines, reservoirs and engine bay components.
From £29.99
Exoracing Titanium or Carbon Exhaust Wrap
Helps keep exhaust heat contained, protecting brake lines and nearby components.
From £24.99
What happens when brake fluid boils?
When brake fluid boils, vapour bubbles form inside the braking system. Unlike fluid, vapour compresses when you press the brake pedal.
This is why the pedal can suddenly feel soft, long, or even drop much further than normal.
This is not the same as normal brake pad fade.
The pedal fade usually feels like the brakes are working less effectively, even though the pedal still feels firm. Boiled brake fluid usually gives you a soft pedal because the hydraulic system can no longer transfer pressure properly.
This is why boiled brake fluid feels so dangerous. The car may feel fine one moment, then suddenly lose braking performance once the fluid gets too hot.
Signs that your brake fluid needs to be changed
There are several signs that your brake fluid needs to be changed. One of the most common signs is a spongy brake pedal, which can indicate air in the brake lines or that the brake fluid is contaminated.
This could be a few things: a leak in the line, it wasn't bled correctly beforehand, or it has naturally gathered air over time. Another sign is a brake pedal that goes to the floor, indicating a leak in the braking system or that the brake fluid is low.
The last sign is when bleeding brakes; if you notice the fluid isn't coming out clear, it may be worth a full flush. The discolouration could be dirt, rust or anything in between, but it will still degrade the brake fluid's performance as it's now contaminated.
If you notice either of these signs, inspecting your brake system as soon as possible is essential. Ignoring these signs can lead to brake failure and put you and other drivers at risk of accidents.
When we boiled the fluid on our AWD Civic, the first sign was a soft pedal under hard braking. Within seconds, braking performance dropped dramatically.
This is exactly how brake fluid failure happens. It is sudden and unpredictable, which is why prevention is far more important than reaction.
How to check your brake fluid
Checking your brake fluid is a simple task that can be done at home with little to no tools. To check your brake fluid, follow these steps:
- Park your car on a level surface and turn off the engine.
- Open the bonnet and locate the brake fluid reservoir. It's usually located on the driver's side of the engine compartment and is labelled "brake fluid."
- Check the level of the brake fluid. The reservoir should be filled to the "max" line. If it's below the "min" line, you'll need to add more brake fluid.
- Check the colour of the brake fluid. Fresh brake fluid should be clear or slightly yellow. If the brake fluid is dark or brown, it's time to change it.
- Replace the brake fluid reservoir cap and close the bonnet.
Best practices for maintaining brake fluid
Brake fluid maintenance is not just about topping up the reservoir.
The goal is to keep the fluid clean, dry, heat-resistant and suitable for how the car is used.
Firstly, it is recommended that you regularly check and replace your brake fluid as needed. This will help ensure that your brake fluid remains at an appropriate level and can effectively transmit hydraulic pressure.
Additionally, it's crucial that you only use the type of brake fluid that your vehicle's manufacturer recommends. Mixing different types of brake fluid can have negative consequences and should be avoided at all costs.
To further maintain the quality of your brake fluid, it's essential to always keep the brake fluid reservoir cap tightly secured. This will prevent contamination from entering the brake fluid and compromising its effectiveness.
Lastly, if you need more confidence in doing this yourself, it is highly recommended that a professional mechanic inspect your brake system regularly.
They will be able to identify any potential issues early on and help ensure that your braking system remains in top condition. Following these best practices can help ensure your vehicle remains safe and reliable on the road.
Motul RBF 660 Factory Line 500ml
The fluid upgrade we would push first for hard use, track days and boiled fluid problems.
From £22.99
Motul RBF 700 Factory Line 500ml
A premium racing fluid option for serious track and motorsport brake temperatures.
From £25.99
Exoracing Silicone Fibreglass High Temperature Heat Sleeve 0.5m
A simple way to protect brake lines, clutch lines and wiring during a brake fluid service.
From £14.99
Exoracing Gold and Silver Heat Reflective Tape
Useful around bulkheads, reservoirs and exposed areas where radiant heat is the problem.
From £29.99
Brake Fluid FAQ's
There are several myths and misconceptions surrounding brake fluid. Here are a few common ones debunked:
Q: Is Motul RBF 660 worth it for road use?
A: If the car is only used gently on the road, standard DOT 4 may be enough. If the car is driven hard, used on track, has upgraded brakes, or has previously boiled fluid, Motul RBF 660 is a worthwhile upgrade because of its much higher boiling point.
Q: Can heat from the engine bay boil brake fluid?
A: Yes, especially when brake lines run close to exhaust manifolds, turbochargers or downpipes. K swapped Civics are a good example, as the rear-mounted exhaust manifold can sit close to the brake lines and proportioning valve.
Q: Should I upgrade brake fluid or brake pads first?
A: Ideally, both should match how the car is used. Better pads create more heat and need fluid that can cope with that heat. If your pedal goes soft, fluid is a major suspect. If the pedal stays firm but the car will not slow properly, the pads may be overheating.
Q: How often should track cars change brake fluid?
A: Track cars should have brake fluid checked and changed more often than normal road cars. Many people change it before a track day or several times per year, depending on use, fluid choice and brake temperatures.
Q: What is the difference between brake fade and boiled brake fluid?
A: Brake fade usually means the pads or discs are too hot and the car no longer slows down properly. Boiled brake fluid usually gives a soft, long, or sinking pedal because vapour has formed inside the hydraulic system.
High-performance brake fluid brands
Numerous brake fluid brands are available, from standard options to those designed for racing. Obtaining brake fluid nowadays is a breeze, with various sources available.
However, for those seeking high performance on the road or track, we strongly recommend these top 3 brake fluid brands that boast higher wet and dry boiling points than conventional brake fluids.
Motul
Motul is our go-to brand for brake fluid, and we always keep multiple bottles of their products on the shelf.
Their extensive line of brake fluids includes options for DOT 3, DOT 4, and DOT 5.1, as well as Racing Brake Fluids such as the Motul RBF 660, which is by far the most popular fluid we sell.
Whether your car is fitted with ABS or ESP systems or you're driving in normal or extreme conditions, Motul has the perfect solution for your specific needs.
PBS Performance Brake Solutions
PBS has already made a name for itself in the racing industry with its Protrack and Prorace brake pads.
Their two lines of racing brake fluid, the 600max and 660max, have also been recognised for their superior performance.
These fluids have higher boiling points than the standard DOT4 fluid, similar to the Motul equivalent. Racing enthusiasts can rely on PBS for their well-stocked products, easily accessible in just a few days.
Choose PBS for high-quality racing products that won't let you down.
To learn more, we have a performance brake pads article outlining everything you need to know.
Mobil
It's important to note that Mobil is a reputable brand in the oil and lubricant industry with a long-standing history and a public trading status.
Their brake fluid is safe, meets all necessary specifications, and is offered at a reasonable price.
Their products are widely available in major retail stores and outlets such as Halfords, making it a brand worth considering despite not being carried by us.
Conclusion
Brake fluid is not just a service item. It is one of the main things protecting you when the brakes get hot.
If your car is only used gently on the road, a quality DOT 4 fluid may be enough. But if you drive hard, do track days, run upgraded pads, or have already experienced a soft pedal, upgrading to a higher boiling point fluid such as Motul RBF 660 is a smart move.
Just remember that brake fluid is only part of the solution. If your brake lines are close to exhaust heat, you also need to protect them with a heat sleeve, reflective tape, or exhaust wrap where suitable.
That combination of better fluid and better heat control is what gives you a braking system that stays more consistent when the car is being pushed hard.
If you are unsure which fluid or heat protection setup is right for your car, please contact us, and we will help point you in the right direction.