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In fact, it is arguably the most important fluid of any type in your passenger vehicle.

Because most modern braking systems rely on hydraulics, the brake fluid is the vital connection between your brake pedal and the calipers that press the brake pads against each of the four wheel discs. Without the proper level of fluid, or if the fluid is of the wrong type, or if the brake fluid is overdue this critical safety function can be severely compromised.

Just as there are many different types and vintages of vehicles, there are different types of brake fluid. These are identified through a fairly simple naming hierarchy associated with each fluid’s operating characteristics – the most important of which are the “dry” and “wet” boiling points.  The dry boiling point is the temperature at which brand-new fluid, straight from a previously unopened container, will reach boil. The wet boiling point is measured after the fluid has absorbed 3.7% of its volume in water.  This second figure is important because most brake fluids are like a huge liquid sponge – they absorb any moisture within their operating environment. That’s a bad thing in terms of brake system performance, which we’ll explain later.

The commonly available types of brake fluid are:

  • DOT 3: 205° C (dry) and 140° C (wet)
  • DOT 4: 230° C (dry) and 155° C (wet)
  • DOT 5.1:  270° C (dry) and 180° C (wet)

Always follow the vehicle manufacturer’s recommendation of fluid type when adding to or replacing brake fluid. Using the wrong type could degrade brake system performance and, in some cases, damage a variety of system components.


The global adoption of hybrid (HEV) and battery-electric (BEV) vehicles represents a seismic shift not only in consumer preference but also the design and operation of multiple vehicle systems, including the braking system. The characteristics of these new cars, including their use of regenerative braking technology, places a variety of additional demands on brake fluid, which is why FERODO has introduced DOT 5.1 EHV brake fluid specifically for these vehicles.

Unlike vehicles powered by internal combustion engines (ICE), HEVs and EVs can regenerate their batteries using the resistance within their electric motors as the vehicle decelerates. ICE vehicles also have built-in resistance – in the form of the engine and transmission – that helps provide braking action when the accelerator pedal is released. The difference in the latter case is that this energy is not recaptured and fed to a battery pack.

Here’s how regenerative braking affects overall brake system operation: When the HEV or EV battery pack is fully charged, the regeneration function is automatically disengaged, which places all of the vehicle’s braking requirements on the conventional hydraulic brake system. Additionally, the heavy battery packs used in these vehicles exert additional load on the conventional braking system, often resulting in higher operating temperatures. 

High braking temperatures can also be an issue when the regenerative system is active. For example, intermittent use of regenerative braking can prevent the vehicle’s brake pads and discs from gradually reaching their normal range of operating temperatures. This can lead to longer-than-normal stopping distances in some cases. And that’s a big deal since even an extra meter can be the difference between an expensive accident and a near-miss.

DOT 5.1 brake fluids, including FERODO DOT 5.1 EHV, address each of these scenarios by offering higher dry and wet boiling points - 274° C (dry) and 184° C (wet).


Another side effect of regenerative braking is the expectation of reduced wear on the vehicle’s conventional brake system, including pads, discs and fluid. (Brake wear varies by type/weight of vehicle, terrain and driving style, among other factors) Unfortunately, this can mean fewer chances for a qualified technician to inspect key system components and correct potential issues before they cause problems on the road.

As one example, corrosion becomes an increasing concern as brake fluid ages. Another issue can arise as various electrical currents within a vehicle come into close proximity to brake hydraulics. Even a sealed brake system can absorb electrical current, which can damage a variety of system components. For these and other reasons, FERODO DOT 5.1 EHV brake fluid is less conductive than other fluid types and offers added lubricity to help protect other system components.

Brake fluids also are affected by low temperatures. Some fluids gradually increase lose viscosity – becoming thicker and reducing their flow properties – as temperatures drop.  Because the conventional brake system is based on hydraulics, it is important for fluids to remain liquid and free flowing under all conditions. FERODO DOT 5.1 EHV maintains outstanding viscosity even at temperatures as low as -40° C.



We’ve already explored the “huge liquid sponge” effect of all brake fluids. While it is virtually impossible to prevent these fluids from absorbing water, the rate of absorption and the fluid’s reaction to water can vary. FERODO DOT 5.1 EHV fluid features a chemistry that slows the decline in the fluid’s boiling point.

Here’s why boiling point matters: Brake fluid cannot be compressed. Therefore, when you press the brake pedal, the fluid’s full volume applies pressure on the calipers, pads and discs for solid, precise brake performance. As water content increases, the fluid’s boiling point decreases, making it more likely that within the mixture pops up some vapor or gas bubble. When this occurs, the water is converted to a compressible vapor, leading to reduced hydraulic pressure within the system, degraded braking power and, in many cases, a “spongy” brake pedal.



Remember, the DOT designation of brake fluid used in your vehicle is dictated by the vehicle manufacturer. Some fluid types are not interchangeable, so be sure to check which one is correct for your vehicle before adding even a small quantity to the brake system.

Never reuse a container of brake fluid that has not been properly sealed. The fluid could be highly contaminated with water.

If you’re driving an HEV or EV, it’s better to use a DOT 5.1 fluid such as FERODO DOT 5.1 EHV, which is formulated specifically for the unique demands of vehicles equipped with regenerative braking technology and other features/characteristics that can affect brake system maintenance cycles.

One last, important point, on new FERODO DOT 5.1 EHV brake fluid: It is water soluble, biodegradable and offers low environmental toxicity. After all, the Earth, like your vehicle’s brake fluid, is a giant sponge, and we need to be careful about what it absorbs!

Click here to learn more about FERODO DOT 5.1 EHV brake fluid – one more example why it’s important to always “Choose the power of performance” that comes with every FERODO product.


The content contained in this article is for entertainment, informational and promotional purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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