What does brake fluid do? Brake fluid is a vital part of your hydraulic braking system — but what exactly is brake fluid, and what does it do? Does brake fluid go bad or need to be changed? We’ve got the answers below, including advice on when to exchange your brake fluid and four signs that your brake fluid level is too low.
What Is Brake Fluid?
Brake fluid is a type of hydraulic fluid used in hydraulic braking systems to transfer force into pressure, bringing the vehicle to a complete stop when the brakes are pushed in. Similar to your engine, modern braking systems require routine flushes and replacements of the brake fluid at designated intervals.
Brake fluid is one of the most critical fluids in your car for a few main reasons:
Brake Fluid Helps Your Car Stop
The relationship between brake fluid, hydraulic braking system, and vehicle motion is a great real-life example of Pascal’s law. This principle states that when a fluid experiences any type of pressure change in an enclosed space, the pressure is transmitted equally in all directions.
When you press your foot on your brake pedal as you approach a red light, a rod forces a piston into the cylinder, creating new pressure throughout the entire hydraulic system. This newly added pressure is distributed throughout the system by forcing brake fluid throughout the brake lines into the caliper pistons.
The pistons then apply the force to your brake pads, applying force to the spinning tire rotors and slowing down the entire vehicle.
Brake Fluid Prevents Rust & Corrosion
The majority of brake fluids are hygroscopic meaning that its great at absorbing moisture. This means that not only does it lubricate all the moving parts and components of your car’s braking system, but the fluid also soaks up any moisture in your system to prevent corrosion and rust.
How does water get there in the first place? Typically, water can enter your system via leaks in your brake lines or broken seals in the master cylinder or caliper.
What Does Brake Fluid Do?
First, the force from your foot is enhanced by a vacuum booster just behind the pedal. This boosted force activates the master cylinder, which pushes pressurized brake fluid into the brake lines. The harder you press on the brake pedal, the more pressurized the fluid becomes, which increases the stopping force applied by the brakes.
From there, the brake fluid flows through the brake lines until it reaches the caliper (or wheel cylinder on drum brakes) on each wheel. At that point, the pressurized fluid forces a set of pistons to push the brake pads against a spinning rotor.
This works because liquid — including brake fluid — is incompressible, meaning it can act as a solid force under pressure, even though it moves as a liquid. When the brake pads rub against the rotor, the friction causes the wheels to slow and eventually stop.
Impressively, all of this happens in the blink of an eye. Yet if your brake fluid has gone bad or is too low, your brakes won’t be as responsive — or worse, they may not work at all. That’s why it’s crucial to have your brake fluid routinely checked.
What Are The Types Of Brake Fluid?
Brake fluid comes in a few different forms: DOT 3, DOT 4, DOT 5, and DOT 5.1. DOT 3, DOT 4 and DOT 5.1 are glycol-based fluids that absorb water and continue to be the most commonly used in modern vehicles.
DOT 5 is a silicone-based fluid that does not absorb water and is typically used in classic cars and vehicles that need non-petroleum-based brake fluids.
Typically, additives are blended into the brake fluid, usually designed to prevent corrosion, rust, or general wear, some additives act as acid neutralizers or pH balancers.
What Type of Brake Fluid Does My Car Need?
The best way to figure out what type of brake fluid you need for your car is by referring to the car’s owner manual provided by the manufacturer or the master cylinder reservoir cap.
Different Types of Brake Fluid
Knowing what brake fluid does and how it helps with the operation of the system as a whole is a good first step as a car owner. It’s just as important to know the difference between kinds of brake fluids, which are known by different Department of Transportation (DOT) numbers.
The three main types of brake fluid include:
- DOT3: This is one of the most common types of brake fluid. It has a glycol base and a lower boiling point. It also can absorb water from the air. Most vehicles can use DOT3 brake fluid.
- DOT4: DOT4 braking fluid is similar to DOT3, but with a higher boiling point. This makes it a more efficient fluid. It’s also very common and can be used in a range of vehicles.
- DOT5: When looking at DOT3 or DOT4 versus DOT5 brake fluid, the biggest difference is that DOT5 is a silicon-based fluid instead of glycol. It doesn’t absorb water like the others and has the highest boiling point. It’s not commonly used on road vehicles.
When Do I Need To Change My Brake Fluid?
Along with the type of brake fluid, your car’s owner manual will indicate how often you need your brake fluid flushed and replaced. In general, brake fluids flushes are recommended every 30,000 miles or two years.
At the end of the day, the frequency of your brake fluid changes depends on your driving habits and braking patterns. Things like constant stop-and-go traffic, sudden braking, and increased mileage can all contribute to needing brake fluid service sooner than later.
Signs Of Low Brake Fluid
If your brake pedal is “softer” than usual, or sinks toward the floor without much resistance, you may be low on brake fluid. When there is less brake fluid to compress, the pedal becomes “squishy” and requires more force from your foot to properly pressurize. This means your brakes are not operating effectively and should be serviced as soon as possible.
Oily puddle under the car
If you notice a puddle of liquid forming beneath your car that is slightly oily to the touch, your brake system may be leaking. There are several places brake fluid can leak from, including rubber hoses, around the calipers, and from the master cylinder. Brake fluid leaks should be fixed promptly to ensure your safety.
ABS warning light
An illuminated ABS warning light on your dashboard may mean your brake fluid is running low. The Anti-Lock Brake System requires sufficient brake fluid levels to work correctly. If the light constantly comes on and off as you drive, you may have a leak.
However, if the Service Brakes warning light is on, you could be dealing with something more serious. Your safest bet is to get a professional inspection whenever a dashboard light or warning message pops up in your vehicle.
Old brake pads
As brake pads wear down, they become thinner and must be pushed farther to rub against the rotor. The thinner the brake pads, the more brake fluid it takes to fill the calipers and squeeze the brake pads.
If your brake pads haven’t been serviced in a long time or are making a whining noise (a possible indication that they need replacing), then it’s a good idea to get your brakes serviced — and maybe replace your brake fluid while you’re at it!
How Do I Check My Brake Fluid?
Checking your brake fluid is very simple and can be completed virtually anywhere. It’s important to remember that brake fluid is toxic and should be handled with care.
Follow these simple steps to check your car’s brake fluid:
- Locate your car’s brake master cylinder. This is the brake fluid reservoir and is usually a simple plastic container on the driver’s side of the vehicle, up against the firewall.
- Check the fluid level by either inspecting the side of the reservoir and making note of the fluid level compared to the fill line. If your vehicle is older, you will need to remove the metal cap, paying special attention to not letting any type of debris fall in. Once opened, make note of the line marked inside the reservoir.
- If the fluid level is low, you can add brake fluid to the full line to provide some temporary relief and save your system from major damage.
- Replace the cap or top and close your hood.
It’s important to note that any type of drop in your vehicle’s brake fluid usually indicates that your brake system requires maintenance or there may be a leak in your line. Either way, it’s important to have your car and brakes inspected as soon as possible.
Does Brake Fluid Go Bad Or Need To Be Changed?
The short answer is yes. Brake fluid does go bad and should be changed following the recommendations on your owner’s manual, or as recommended by a technician based on copper level testing results.
There are a few reasons why your brake fluid loses effectiveness over time. The first is that brake fluid is hygroscopic — a fancy word that means it readily absorbs moisture from the air. Even though your brakes are supposed to be part of a closed system, it’s impossible to keep small amounts of moisture from mixing with your brake fluid.
This is usually due to microscopic holes in rubber hoses, imperfect seals, or when the brake fluid cap is left open longer than necessary.
Over time, exposure to the air can increase the water content inside your brake fluid. Even a small amount of water contamination can cause your brakes to start losing effectiveness since water decreases your brake fluid’s boiling point.
Another reason your brake fluid may need changing is due to corrosion in the brake lines. Brake fluid contains corrosion inhibitors and antioxidants that help keep important parts of the brake system working smoothly.
Over time, however, these inhibitors break down, which leads to metal corrosion and the build-up of contaminants that disrupt the flow of brake fluid. Excess water can also cause metal parts to corrode.
Avoid driving around with contaminated brake fluid by visiting your local car care experts for a free brake inspection, including brake fluid testing!
Depending on their findings, our certified technicians may recommend a brake fluid exchange, which involves removing and replacing the brake fluid in your hydraulic braking system. Always consult your owner’s manual for a more exact recommendation on brake fluid services.
How To Add Brake Fluid
- Determine the location of the brake fluid reservoir
- Look inside the reservoir to determine the current fluid level. If it’s at the “full” mark, close the reservoir and mark the date of the inspection in your maintenance log.
- If the fluid level is below the “add” line, have your disc brakes checked. As disc brakes wear down, the fluid is displaced into the brake calipers. Most brake fluid reservoirs are designed so you should not have to add or “top off” the fluid. If the fluid looks dark, have a mechanic test and inspect the fluid.
What brake fluid to use?
The best way identify what type of brake fluid your car needs is to check your owner’s manual. You can also check the master cylinder reservoir cap to identify the type of brake fluid used in your vehicle.
What happens if the brake fluid is low?
This is a sign you need a brake fluid change. Low brake fluid will cause air to fill the gaps in your brake line—leading to soft brakes. Spongy brake pedals can be both terrifying and dangerous—especially if you do not get them serviced at the first sign of an issue.
What happens if you don’t change brake fluid?
It will cause excessive heating and make the fluid boil, rendering your brakes inefficient. Allow the water to build up and remain in the vehicle, and internal corrosion may damage the master cylinder, calipers, brake lines, and other components.
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