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Brake Fluid: What It Does, Types, Changing, Costs, FAQs

April 9, 2021

Think of your brake system as a living organism. 

If the master cylinder is the “heart,” then brake fluid is the “blood” pumping through the brake lines. Without that brake fluid, your brakes won’t work. 

So, what exactly is brake fluid, and why is it so important? 

In this article, we’ll dive into everything about brake fluid, including how it works, problems associated with it, and the easiest way to remedy brake fluid problems

This Article Contains

What’s Brake Fluid And What Does It Do?

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

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

So how does brake fluid work? 

When you depress the brake pedal, brake fluid transfers that force to your disc brakes (or your drum brake) to stop the wheels from spinning.

Here’s a detailed rundown of what happens: 

Now that you know how it works, let’s look at the key characteristics of brake fluid. 

4 Brake Fluid Characteristics You Should Know

There are certain brake fluid characteristics that you need to know to understand it better: 

1. It Has A High Boiling Point

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

Brake fluid has types two boiling points:

The dry boiling point is always higher as it represents the boiling point of new brake fluid. The wet boiling point is what it’s expected to be after some moisture contamination in real conditions.

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

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

2. Maintains Constant Viscosity

Viscosity indicates how thick a liquid is and its ease of flow.

Brake fluid needs to maintain constant viscosity over a wide temperature range as it needs to perform the same way in different temperature conditions.

This is especially important in anti-lock braking systems (ABS) or traction and stability control systems, as these systems use microvalves and need rapid activation.

3. It Is Corrosive

Pure brake fluid is corrosive, which is why commercial brake fluid usually has corrosion inhibitors added to it. This prevents corrosion in brake parts like the caliper or master cylinder as moisture enters the brake system.

The additive package depends on the manufacturer and may not only include corrosion inhibitors but have anti-wear and anti-foaming qualities too.

Note: Silicone fluid is less corrosive to paintwork, unlike a glycol-based fluid.

4. Has Low Compressibility

Compressibility refers to how something (like liquid) reduces in size under pressure. 

Brake fluid has to maintain a low level of compressibility (unlike a sponge, which is highly compressible). Otherwise, your brake pedal feel might keep changing, and the hydraulic pressure generated won’t be applied consistently on the brake rotor.

Next, what are the different types of braking fluids out there?

5 Types Of Brake Fluid

When it comes to types of brake fluid, you’ll probably notice different “DOT” labeled fluids. 

These are defined by the Federal Motor Vehicle Safety Standards (FMVSS), where “DOT” stands for the US Department of Transportation. There are also international standards defined under ISO 4925 and grades determined by the SAE (Society of Automotive Engineers). 

However, the DOT brake fluid certification is widely used in many countries, so we’ll stick to that. 

1. DOT 3

The DOT 3 fluid is an affordable glycol-based fluid that’s the most common type of brake fluid used today. 

Fresh DOT 3 fluid is an amber color and has a dry boiling point of 401oF. But when fully degraded, the boiling point drops to 284oF. 

Glycol fluid is highly corrosive and will remove paint. 

2. DOT 4

DOT 4 brake fluid is glycol-based fluid like DOT 3, but with additives to increase its minimum boiling point. 

There are different types of DOT 4 brake fluid, like DOT 4 Plus or DOT 4 Racing. 

All DOT 4 fluids have a higher boiling point than DOT 3 fluid, starting at 446oF. 

DOT 4 brake fluid is usually amber like DOT 3, except for DOT 4 Racing, which has a blue additive. While DOT 4 fluid can handle higher temperatures, it also needs to be changed more frequently. 

3. DOT 5

DOT 5 fluid is entirely different from DOT 3 and DOT 4 as it’s a silicone fluid. 

Because it’s silicone-based, it doesn’t attract water, prevents rust, and doesn’t corrode paint. It has a high dry boiling point of 500oF and is typically purple to differentiate it from DOT3 and DOT4. 

However, it’s about 4x more pricey than DOT 3 and is outperformed by some specialty DOT 4 fluids. It can also become foamy with air bubbles that can be harder to bleed out, which is why it’s not recommended for vehicles with an ABS. 

4. DOT 5.1

DOT 5 and DOT 5.1 sound very similar, but they’re actually very different brake fluids.

DOT 5.1 brake fluid is glycol-based, like DOT 3 and DOT 4, but has a similar boiling point to DOT 5 fluid. It’s also around 14x more expensive than DOT 3! 

You’ll likely find DOT 5.1 brake fluid used for high-performance vehicles, race cars and also in heavy-duty applications because of its high boiling point. 

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 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 a mineral oil.

Now that you know the different types of brake fluid, let’s go over some of the potential problems you could encounter:

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

1. Fluid Leaks

A brake fluid leak is a mechanical problem, so it can occur with any type of brake fluid. 

When there’s a leak, hydraulic pressure in the brake line drops, and air bubbles may enter the braking system — affecting brake performance and possibly leading to brake failure. 

A leak can happen anywhere, from torn brake hoses to a failed master cylinder. If your brake pedal sinks to the vehicle floor, that’s a strong indication that you’re losing brake fluid.

2. Moisture Creeping In

Brake fluid is generally hygroscopic, meaning it absorbs moisture. Water contamination degrades brake fluid over time, decreasing its boiling point and brake performance. 

Though the brake system is a closed system, moisture can still seep through microscopic holes in rubber brake hoses or via imperfect seals. It even happens when the brake fluid reservoir is left open too long. 

3. Rusting

Even though brake fluid has corrosion inhibitors, those additives will 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 failure in the vehicle’s brakes — like a stuck caliper, for instance. 

4. Fluid Contamination

Not only does brake fluid go bad over time with moisture exposure, but components in the brake system will deteriorate too. Pieces of rubber from a wheel cylinder or bits of debris can also end up in the brake fluid, making it sludgy and reducing brake fluid performance.

Now that you know what brake fluid is and what can go wrong with it, we’re sure you have a few questions.

Let’s go over some of them:

8 Brake Fluid FAQs

Here are the answers to some FAQs you might have about your vehicle’s brake fluid.

1. When Is A Brake Fluid Change Needed?

For regular day-to-day driving, it’s a good rule of thumb to change your brake fluid at least once in two years. However, always follow the manufacturer’s recommendations if there is one. You will find their recommendations in your vehicle’s owner’s manual.

Once the right amount of time has elapsed, you can always ask your mechanic to change your brake fluid when you get an engine oil change. Just keep in mind that the harder you are on your vehicle’s brakes, the faster you’ll need a change.

2. What’s The Benefit Of Changing Brake Fluid?

Apart from ensuring that your brakes are in perfect condition, you may want to consider the cost benefits attributed to consistent brake fluid upkeep. 

While flushing and replacing your car’s brake fluid may cost around $100 for most vehicles, replacing rusted brake calipers or brake lines (which arise due to brake fluid problems) can be much more expensive.

3. What Are The Signs Of Low Brake Fluid?

Some tell-tale signs of low brake fluid are:

4. Can I Check The Brake Fluid Condition Myself?

Yes, you can. 

But should you? 

That depends. 

You can open the brake fluid reservoir to take a quick look at the vehicle’s brake fluid. 

Old fluid darkens with age and contamination. You can also check the brake fluid level to see if it has dropped below the minimum line. 

But remember — brake fluid is toxic

Also, never leave the brake fluid reservoir open longer than needed because you’re exposing it to air. That’s why it’s always better to get a mechanic to inspect it

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.

5. Can I Change Brake Fluid Myself?

It’s not recommended.

Unless you’re an experienced mechanic, please ask a professional to change the old fluid for you. 

6. Can Different Brake Fluids Mix?

Generally no

Use whatever is recommended for your vehicle. 

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

Glycol-based fluids like DOT 3, DOT 4, and DOT 5.1 are technically compatible with each other. However, it’s always better to get refilled with the same brake fluid. 

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

7. What Is A Brake Fluid Flush?

A brake fluid flush is when all the brake fluid is removed from the brake system using a vacuum and replaced with clean fluid.

8. What Is Brake Bleeding?

Brake bleeding is where just enough new brake fluid is pushed through the brake lines to remove air bubbles. This brake fluid service is slightly different from a brake flush as it’s quicker and cheaper, albeit less comprehensive. 

A brake fluid flush or bleed can cost anywhere between $70-$120, but it can cost more, depending on your car’s make and model. 

Your next question may be:

What’s the easiest way to deal with a brake fluid change or issue?

The Most Convenient Fix To All Your Brake Fluid Issues

Because of the vital role it plays in your braking system, you’ll want your brake fluid adequately serviced. 

That’s why it’s important to find a reliable mechanic for your brake repairs.

Make sure that they:

Luckily for you, RepairSmith is not only easy to contact but also checks all those boxes!

RepairSmith is a convenient mobile vehicle repair and maintenance solution, and here’s why they’re your #1 option:

For an accurate estimate of repair costs, just fill this online form.

Don’t Overlook Your Brake Fluid Needs

Brake fluid is an incredibly important component in your brake system, so don’t overlook it.  

If you’re concerned about the hassle of finding a mechanic or driving to a repair shop to get your brake fluid checked, you don’t need to worry.

Book an appointment with RepairSmith, and an ASE-certified brake technician will be at your driveway, ready to address your brake fluid needs ASAP.