Understanding Brake Fluid

What is brake fluid?

Brake fluid is a crucial component of a vehicle’s braking system, transmitting the force from the brake pedal to the brake calipers, which ultimately slows or stops the vehicle. Choosing the correct brake fluid is essential for performance, safety, and system longevity. There are several types of brake fluid, each with its own characteristics. The most commonly used in road cars are glycol-based fluids: DOT3, DOT4, and DOT5.1.

Brake fluid Basics

Brake fluid needs to perform under extreme conditions—high pressure, varying temperatures, and moisture exposure. Key properties include:

  • Boiling point: This is the maximum temperature brake fluid can reach before it boils causing brake fade and potentially brake failure. Two boiling points are quoted, dry boiling point is for brand new brake fluid, wet boiling point is for fluid that contains 3.7% water by volume.

  • Viscosity: Affects brake responsiveness.

  • Hygroscopic nature: Glycol-based fluids absorb moisture over time, even in sealed systems. This lowers the boiling point and can, in extreme cases, cause corrosion. Manufacturers recommend that brake fluid is changed every 2 years to maintain brake performance.

  • Compatibility: DOT3, DOT4 and DOT5.1 brake fluids are compatible with each other. It wouldn’t be desirable to mix a lower specification into a higher one though.

Now let’s break down the most commonly used types

DOT3 Brake Fluid

  • Base: Glycol ether

  • Dry boiling point: ~205°C

  • Wet boiling point: ~140°C 

  • Hygroscopic: Yes

DOT3 is the most basic glycol-based brake fluid. It’s suitable for standard driving and older vehicles with less demanding brake systems. It has a relatively low boiling point, which can be a concern under heavy braking, such as mountain driving or spirited use. It isn’t really in general usage these days.

DOT4 Brake Fluid

  • Base: Glycol ether

  • Dry boiling point: ~230°C 

  • Wet boiling point: ~155°C 

  • Hygroscopic: Yes

DOT4 is an improvement over DOT3, offering higher boiling points. It’s commonly used in modern cars and is suitable for light performance driving. DOT4 is often recommended for vehicles with ABS and traction control systems, as these systems rely on quick and precise brake actuation. DOT4 is the most widely used brake fluid today.

DOT5.1 Brake Fluid

  • Base: Glycol ether (not to be confused with DOT5)

  • Dry boiling point: ~260°C 

  • Wet boiling point: ~180°C 

  • Hygroscopic: Yes

  • Climate: Has a lower viscosity than DOT3 or DOT4 and is therefore more suitable for cold climates

DOT5.1 is chemically similar to DOT3 and DOT4 but has the highest performance characteristics of the three. It’s ideal for high-performance road cars and some motorsports applications where extreme braking conditions are expected. Despite its high boiling points, it retains compatibility with ABS systems and does not require any hardware modifications from DOT4 systems. It is widely used in colder climates where average temperatures are lower.

DO I Need performance brake fluid?

Performance brake fluid – typically a high-spec DOT4 or DOT5.1 – is formulated to withstand extreme temperatures and pressures found in racing or aggressive driving scenarios. It is desirable when:

  • You drive in mountainous or hilly regions frequently.

  • You tow heavy loads regularly.

  • You participate in track days, autocross, or spirited driving.

  • Your vehicle has a performance brake system or is modified for higher speeds.

Upgrading to performance brake fluid can provide better pedal feel, reduce brake fade, and improve safety during hard braking – but it requires more frequent changes due to faster degradation. 

In the vast majority of cases standard DOT4 brake fluid is recommended for road use. 

Silicone-Based and Mineral Brake Fluids:
Why They’re Rare in Road Cars

  • DOT5 is a silicone-based brake fluid. Unlike glycol fluids, it is non-hygroscopic, meaning it does not absorb water. While this prevents internal corrosion, it also means any water that does enter the system forms pockets that can boil and cause vapor lock. DOT5 is also incompatible with ABS systems and must not be mixed with DOT3, DOT4, or DOT5.1. It’s mainly used in classic cars and military vehicles where moisture absorption is less of a concern. A big advantage with silicone based brake fluid is it doesn’t degrade over time, so where vehicles are infrequently used but need to be ready at short notice – like military vehicles – it will be ready to go. Silicone is more compressible than glycol-based brake fluid and will give a spongier pedal feel. 

  • Mineral-based fluids (used in some historic Citroëns, Rolls Royce and Bentley cars) are non-corrosive and non-hygroscopic but are used in specially designed systems. They are not compatible with most modern braking systems and are rare in typical road vehicles.

Conclusion

For most drivers, DOT4 will suffice, depending on the vehicle’s age and braking demands. For high-performance applications, DOT5.1 or a performance-grade DOT4 offers superior resistance to heat and fade. While silicone and mineral-based fluids have niche uses, they are not suitable for the vast majority of road vehicles due to compatibility and safety concerns. Always consult your vehicle manufacturer’s specifications before switching brake fluids, and when in doubt, err on the side of higher performance with regular maintenance.

bleeding brake fluid

Brake Fluid