What Is Brake Fluid? (Different Types + Characteristics)

Brake fluid plays a vital role in overall braking performance and safety, which means it needs to be checked and replaced regularly.

Interestingly, it comes in several different formulations, each designed for specific braking systems and performance requirements.

Common brake fluid types include:

DOT 3 fluid
DOT 4 fluid
DOT 5 fluid
DOT 5.1 fluid
Mineral oils (LHM)

But what exactly is brake fluid, and why is it important to use the correct type?

Read on to learn everything you need to know about brake fluid — from how it works and its key characteristics to the signs that indicate it may be time for a change.

This Article Contains:

What Is Brake Fluid and What Does It Do?
5 Types of Brake Fluids Available for Cars
4 Brake Fluid Characteristics You Should Know
7 Signs It’s Time for a Brake Fluid Check
3 Other Brake Fluid-Related Problems You Should Be Aware Of
When Is a Brake Fluid Change Needed?
How Much Does a Brake Fluid Change Cost?
4 FAQs About Brake Fluid

What Is Brake Fluid and What Does It Do?

Brake fluid is a hydraulic fluid used in modern hydraulic brake and clutch systems.

Most brake fluids you’ll encounter are glycol-based, but there are also mineral oil-based and silicone-based fluids.

Here’s how they work:

The force from pressing the brake pedal is amplified by a vacuum booster.

The vacuum booster activates the master cylinder, which converts that force into hydraulic pressure by pressurizing the hydraulic fluid in the brake lines.

That hydraulic pressure is transmitted through the brake fluid, engaging the braking mechanism. For example, in a disc brake system, the brake fluid actuates the brake caliper. In drum brakes, the fluid actuates the wheel cylinder.

Finally, in disc brake systems, the brake calipers clamp the brake pads against the brake rotors. In drum brakes, the wheel cylinder pushes the brake shoes against the brake drums, creating friction that slows and stops the wheels.

Aside from engaging your brakes, brake fluid lubricates the moving parts of your brake system. It also absorbs moisture, helping prevent corrosion and rust.

Now that you know how it works, let’s look at the different types of this fluid.

5 Types of Brake Fluids Available for Cars

Using the correct brake fluid type and understanding how it behaves under pressure can help prevent braking issues and costly repairs.

So, let’s look at the different brake fluid types you can use:

1. DOT 3 Fluid

DOT 3 brake fluid is an affordable glycol-based brake fluid. Its dry boiling point is 401°F, but it drops to 284°F when degraded (due to absorbing moisture).

2. DOT 4 Fluid

DOT 4 brake fluid is a glycol-based fluid like DOT 3, but it contains additives that raise its minimum boiling point.

Although there are different types of DOT 4 brake fluid (like DOT 4 Plus or DOT 4 Racing), all have a higher boiling point than DOT 3 brake fluid, starting at 446°F.

3. DOT 5 Fluid

Unlike DOT 3 and DOT 4, DOT 5 fluid is a silicone-based fluid that repels water, preventing rust and corrosion. It has a high dry boiling point of 500°F. However, it can become foamy with air bubbles that are harder to bleed out.

NOTE: The DOT 5 fluid is not recommended for vehicles with Anti-lock Braking Systems (ABS), also known as anti-lock brakes.

4. DOT 5.1 Fluid

DOT 5.1 is a glycol-based brake fluid like DOT 3 and DOT 4, but it has a boiling point similar to DOT 5 fluid (500°F). Because of its high boiling point, DOT 5.1 brake fluid is mostly used in high-performance vehicles, race cars, and heavy-duty hydraulic brake systems.

5. Mineral Oils (LHM)

In 1966, Citroen introduced the LHM (Liquide Hydraulique Minéral) — a mineral oil used in specific Citroen engines and braking systems. Some old Rolls-Royce and Maserati models use it too.

LHM handles temperatures better than DOT 3 and DOT 4, with a dry boiling point of 480°F. There’s also no corrosion because it’s mineral oil.

NOTE: The United States Department of Transportation classifies brake fluid according to the Federal Motor Vehicle Safety Standards (FMVSS). There are also international standards defined under ISO 4925, and specific brake fluid types are also designated by the Society of Automotive Engineers, including the SAE J1703 and SAE J1704.

Next, let’s dive deeper into the nitty-gritty details of brake fluids.

4 Brake Fluid Characteristics You Should Know

Here are the important brake fluid traits:

1. High Boiling Point

Brake fluid is exposed to very high temperatures, so it naturally has to have a high boiling point.

The fluid has two boiling points:

Dry boiling point: The boiling point of fresh brake fluid.
Wet boiling point: This is the fluid’s boiling point after absorbing some moisture.

The dry boiling point is always higher, as it represents the boiling point of new brake fluid. The wet boiling point is the boiling point after the fluid absorbs 3.7% moisture by volume.

If the temperature exceeds its boiling point, the brake fluid will vaporize into gas, impacting its ability to transmit hydraulic pressure and reducing braking capacity.

The older your car’s brake fluid is, the lower its boiling point will likely be (due to moisture exposure), and the faster it will vaporize at extreme temperatures.

2. Constant Viscosity

Viscosity indicates how thick a liquid is and how easily it flows.

To ensure reliable performance in all weather conditions, brake fluid needs to maintain constant viscosity over a wide range of temperatures. This is especially important for anti-lock brakes or traction and stability control systems, which use microvalves and need rapid activation.

3. Corrosiveness

Brake fluid is corrosive to paint and rubber, but it can also corrode metal if contaminated with water.

Commercial brake fluid usually contains a corrosion inhibitor. This prevents corrosion of brake parts, such as the caliper or master cylinder, when moisture enters the brake system.

The additive package varies by manufacturer. It could include a corrosion inhibitor, an anti-wear agent, an anti-foaming agent, and an anti-vapor lock formulation.

NOTE: Silicone fluid is less corrosive to paintwork than glycol-based fluid.

4. Low Compressibility

Compressibility refers to how something reduces in size under pressure.

Brake fluid must maintain a low level of compressibility. Otherwise, the brake pedal response may become inconsistent, and the hydraulic pressure generated won’t be reliably applied to the wheels.

In the next section, we’ll review some red flags that could signal a brake fluid issue.

7 Signs It’s Time for a Brake Fluid Check

Suspect your brake fluid is past its prime?
Watch out for these symptoms:

Lit ABS warning light: If you notice an illuminated ABS dashboard light, consider getting a brake fluid inspection. If the issue isn’t due to the brake fluid, you might have a faulty ABS system.

Soft brake pedal: If your brake pedal feels soft, loose, or spongy when you step on it, you may have contaminated or low brake fluid.

Reduced brake responsiveness: Dirty or low brake fluid can cause a sudden change in brake performance or even brake failure. However, low brake responsiveness could also be due to damaged brake rotors or brake pads.

Brake fluid leaks: An oily puddle beneath your car, especially around a wheel or brake line, can indicate a brake fluid leak that may cause brake failure.

Burning smells: A strong burning smell or smoke around the wheels can mean your brakes are overheating.

Thin or worn brake pads: As brake pads wear down, the calipers need more brake fluid to operate properly, which can lower the fluid level. And because worn pads can also produce symptoms that mimic a low brake fluid level, it’s important to have a skilled mechanic inspect the brakes to identify the actual cause.

Strange noises: If the brake fluid is low, old, or contaminated, you may hear squealing, grinding, or unusual hissing noises when stepping on the brake pedal.

Besides the points we’ve covered, there are other reasons to have your brake fluid checked.

3 Other Brake Fluid-Related Problems You Should Be Aware Of

Here are some brake fluid-related issues you might face:

Moisture creeping in: Moisture can enter the brake system through tiny holes in rubber brake hoses, imperfect seals, or if the brake fluid reservoir is left open. Since brake fluid is generally hygroscopic, it absorbs moisture. This degrades brake fluid over time, decreasing its boiling point and brake performance.

Fluid contamination: Moisture contamination in brake fluid eventually deteriorates brake system components. Pieces of rubber from a wheel cylinder or other debris can also end up in the brake fluid, making it sludgy and reducing its performance.

Rusting: Corrosion inhibitors in brake fluid break down over time. When this happens, moisture in the brake fluid may cause rust, blocking small passages in the brake hardware. Rust can create problems in the vehicle’s brake system, such as a stuck caliper.

The best way to prevent these issues is to regularly change your brake fluid.

When Is a Brake Fluid Change Needed?

For regular driving, you should change your brake fluid or get a brake fluid flush (a.k.a. brake fluid exchange) at least once every two years or every 30,000 miles.

To get the exact information on the service intervals, refer to the owner’s manual. Remember, the harder you are on your vehicle’s brakes, the faster you’ll need a change.

Next, let’s see how much you’ll need to spend on a brake fluid exchange.

How Much Does a Brake Fluid Change Cost?

A brake fluid change costs about $80 to $130, depending on your brake fluid type, location, and labor charges.

However, replacing rusted brake calipers or brake lines (which result from brake fluid problems) can be much more expensive. To avoid such wallet-denting repairs, it’s best to get the brake fluid service done at the recommended intervals.

Still have some doubts?
We’ve got you covered.

4 FAQs About Brake Fluid

Here are answers to queries you might have about brake fluid:

1. What Is the Benefit of Changing Brake Fluid?

Changing brake fluid routinely ensures your brakes work perfectly and helps you avoid additional service costs for rusted or damaged brake parts.

2. Can I Check the Brake Fluid Condition Myself?

Yes, you can quickly open the brake fluid reservoir under the hood to examine your vehicle’s brake fluid.

How do you tell that the brake fluid is still in good condition?

Old fluid darkens with age and contamination. So, a darker color means it’s likely time to get a brake fluid change. While at it, you can also check the brake fluid level to see if it has dropped below the minimum line.

However, brake fluid can be toxic if ingested, inhaled, or if it comes into contact with the skin or eyes. Leaving the reservoir open longer than needed can also expose brake fluid to moisture in the air.

For your peace of mind, always have a mechanic check the brake fluid level and condition. Even if your brake fluid hasn’t turned sludgy or the fluid level is fine, only a professional can test it for moisture contamination and tell you if a change is required.

3. Can I Change the Brake Fluid Myself?

No, you shouldn’t attempt a DIY brake fluid change. While you can buy brake fluid at your preferred store and change it according to the owner’s manual, it’s best to leave this job to a professional.

Why?
An expert will perform the correct brake fluid exchange and use the right brake fluid. This will help ensure the proper functioning of your braking system.

4. Can I Mix Different Brake Fluids?

Generally, no. You should stick to the fluid recommended for your vehicle.

However, if you’re in an emergency and need to top off your brake fluid due to a leak, but your regular DOT fluid isn’t available, there are some options.

Glycol-based fluids like DOT 4 and DOT 5.1 are technically compatible. However, you should only add fluid of a higher grade (e.g., DOT 4 to DOT 3, not the other way around).

CAUTION: If you’re using DOT 5 or LHM, don’t try mixing them with anything else, as they’re incompatible.

For more information on brake fluids, check out some of our related posts covering:

Fix Your Brake Fluid Issues with AutoNation Mobile Service

Since brake fluid plays a vital role in your braking system, you’ll want to have it regularly checked and replaced by professionals.

But who can you trust for this job?
The answer is AutoNation Mobile Service.

With us, you don’t have to visit a shop — our mechanics will come and resolve your brake fluid issues right from your driveway. We also offer a 12-month, 12,000-mile warranty for your peace of mind.

Contact us so we can resolve all your brake fluid-related issues hassle-free.