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 the liquid chemical solution used in the hydraulic braking systems of modern cars. It is designed to amplify your foot’s force on the brake pedal and turn it into pressure on your car’s brakes. Without brake fluid, it would take a lot more than just your foot to stop your car!
Brake fluid must be the right viscosity.
Brake fluid must be formulated just right to be effective. First, it needs a constant viscosity across wide temperature ranges (viscosity refers to how thick a liquid is and how easily it flows). The best way to determine the right brake fluid type for your car is to check the owner’s manual or ask your nearest car care experts. That way, you can ensure you’re getting brake fluid that has the right viscosity and base (most brake fluids are either glycol- or silicone-based, and the two types should never be mixed)!
Brake fluid must have a high boiling point.
Second, brake fluid must have a high boiling point. That’s because friction from the brake pads creates a lot of heat — if the heat around the brakes causes the fluid to reach its boiling point, it will be vaporized into gas, and the brakes will lose effectiveness.
Brake fluid should resist corrosion.
Last, brake fluid requires additives that are specially formulated to prevent corrosion in the brake lines.
What Does Brake Fluid Do?
So what exactly does brake fluid do when you step on the brake pedal? 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.
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.
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!
Service When It Works For You
Brake fluid is essential for the safety of you and your vehicle! And it's best to make sure your brake fluid isn’t stopping you from keeping up with your safe driving habits. Drop in for a free brake inspection at your nearest Tires Plus! Our technicians will check out your brake fluid and other essential brake system components, and make recommendations based on what they see — no strings attached!