Blog Car Care Advice What Is Brake Fluid & How to Tell When It’s Low?
Car Care Advice

What Is Brake Fluid & How to Tell When It’s Low?

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While fuel keeps your vehicle rolling, brake fluid helps it stop. 
Without it, your brakes won’t work

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

Read more to learn all about brake fluid, including how it works and the signs that indicate you need a fluid check. 

This Article Contains

What Is Brake Fluid and What Does It Do?

Brake fluid is a hydraulic fluid used in modern hydraulic clutches and brake 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:

  1. The force from pressing the brake pedal is amplified by a vacuum booster.
  2. The vacuum booster activates the master cylinder, which converts that force into hydraulic pressure by pressurizing the hydraulic fluid in the brake line.
  3. That hydraulic pressure is transmitted through the brake fluid, engaging the braking mechanism. This could be a brake caliper (in a disc brake system) or a wheel cylinder (in a drum brake).
  4. The brake calipers clamp a brake pad on each side of the brake rotor (or the wheel cylinder pushes brake shoes on a brake drum), stopping the wheel.

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 brake fluid. 

5 Types of Brake Fluids Available for Cars

The different brake fluid types include:

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 has additives to increase 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 fluid, starting at 446°F. 

But here’s the thing:
While DOT 4 fluid can handle higher temperatures, it also needs to be changed more frequently. 

3. DOT 5 Fluid

Unlike DOT 3 and DOT 4, DOT 5 fluid is a silicone fluid that repels water, preventing rust and corrosion. 

This DOT fluid has a high dry boiling point of 500°F. However, it can become foamy with air bubbles that are harder to bleed out. So, it’s not recommended for vehicles with an Anti-lock Braking System (ABS). 

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 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 US Department of Transportation or DOT brake fluid is defined by the Federal Motor Vehicle Safety Standards (FMVSS). There are also international standards defined under ISO 4925 and brake fluid types determined by the Society of Automotive Engineers, like the SAE J1703 and SAE J1704.

After exploring the different types of brake fluids, let’s review their characteristics.

4 Brake Fluid Characteristics You Should Know

Here are four 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. 

That said, the fluid has 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 the boiling point after the fluid absorbs 3.7% moisture by volume. If the temperature increases beyond 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 (because of moisture exposure), and the faster it will vaporize in extreme temperatures.

2. Constant Viscosity

Viscosity indicates how thick a liquid is and its ease of flow. Brake fluid needs to maintain constant viscosity over a wide range of temperatures to ensure reliable performance in all weather conditions.

This is especially important for Anti-lock Braking Systems (ABS) 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 after water contamination. So, commercial brake fluid usually has a corrosion inhibitor 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. It could include a corrosion inhibitor, anti-wear, anti-foaming, and 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 feel of the brake pedal may change, and the hydraulic pressure generated will not be consistently applied to the wheels.

Next, let’s review some red flags indicating a brake fluid issue.

6 Signs It’s Time for a Brake Fluid Check

Watch out for these symptoms of brake fluid trouble:

  1. 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 or wheel speed sensor.

  2. Brake pedal feels soft: If your brake pedal feels soft, loose, or spongy when stepped on, you may have contaminated or low brake fluid.

  3. Reduced brake responsiveness: Dirty or low brake fluid can lead to a sudden change in brake performance. However, low brake responsiveness could also be due to a damaged rotor or brake pad.

  4. 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.

  5. Burning smells: A strong burning smell or smoke around the wheels can mean your brakes are overheating. If this is paired with a loud screeching or squealing sound, you need to get the brake fluid replaced and the brakes serviced to avoid brake failure.

  6. Thin or worn brake pads: Worn brake pads can be a sign and cause of low brake fluid since they require more fluid to fill the calipers and properly squeeze them. Moreover, thin brake pads typically trigger symptoms similar to low brake fluid, so it’s best to have a mechanic confirm the issue.

Now, there are other reasons to have your brake fluid checked. 
Let’s explore them.

3 Other Brake Fluid-Related Problems

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

  1. 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.

  2. Fluid contamination: Moisture contamination in brake fluid eventually deteriorates brake system components. 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.

  3. 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—like a stuck caliper

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

But how often do you need to do that?
Read more to find out. 

When Is a Brake Fluid Change Needed?

For regular driving, you should change your brake fluid or get a brake fluid flush at least once in two years or every 30,000 miles. For exact intervals for your vehicle, refer to your owner’s manual. But remember, the harder you are on your vehicle’s brakes, the faster you’ll need a change.

You could also ask your mechanic to check your brake fluid when you get an engine oil change. 

Next, let’s see how much you’ll need to spend for this change.

How Much Does a Brake Fluid Change Cost?

A brake fluid change could cost about $80 to $130, depending on your brake fluid type, location, and mechanic’s labor charge.

But remember: 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 on Brake Fluid 

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

1. What’s the Benefit of Changing Brake Fluid?

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

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. That’s why it’s always better to get a mechanic to inspect it. 

And 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. Unless you’re an experienced mechanic or have professional training, it’s best to leave a brake fluid service to an expert. 

4. Can I Mix Different Brake Fluids?

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. However, you should only add fluid of a higher grade (like adding DOT 4 to DOT 3 — not the other way around). Still, it’s best to refill the same DOT brake fluid.

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

Fix Your Brake Fluid Issues with AutoNation Mobile Service

Because brake fluid plays a vital role in your braking system, you’ll want to have it adequately serviced.

That’s why finding a reliable mechanic like AutoNation Mobile Service for your brake repairs is important.
We’re a mobile vehicle repair and car care solution offering competitive pricing, stellar customer service, and a 12-month, 12,000-mile warranty for all repairs.

Contact us to get any brake service or other fluid top-ups, including a power steering fluid or motor oil change, done right from your driveway.