What is brake fluid, and what does it do? Is Brake Fluid Bad? Does it need to be replaced? Here’s everything you need to know about brake fluid and when it is time to change it.
If you are unsure whether your brake fluid is low, here are four signs that will help you determine if you need to change your brake fluid.
What is brake fluid?
In a vehicle, brake fluid is the hydraulic fluid used in both hydraulic brake and hydraulic clutch systems.
Brake fluid is used to transfer the force applied to the brake pedal into pressure that is applied to the front and rear brakes to stop the vehicle. Brake fluid assists in increasing the amount of braking force applied to the brake pads because liquids are incompressible.
What Does Brake Fluid Do?
Brake fluid is critical to the braking system of your vehicle—it acts as a liquid chemical solution that transfers the force created by your foot pressing down on the brake pedal to the brakes via hydraulic braking systems.
When you apply the brakes, extreme temperatures occur due to friction between the pads and the rotors, creating heat. As this heat builds up in the brake hydraulic system, moisture in the system begins to condense.
Brake fluid absorbs this moisture before it can boil away and cause brake failure. In addition, brake fluid provides lubrication for the moving parts in the braking system and protects them from rust and corrosion.
For use in a vehicle, brake fluids undergo tests to ensure they will stay liquid at low temperatures and resist boiling at high temperatures. Brake fluid must also be compatible with other brake fluids and parts of the braking system, as well as effectively control corrosion within the brake system.
How Brake Fluid Works?
The role of brake fluid is similar to that of electrical impulses sent from your brain to your muscles. When you apply the brake pedal, brake fluid will travel through the brake system from the brake pedal to the caliper and pad. A mechanic can only determine if the brake fluid is low.
When applying pressure to your brake pedal, you force a piston or pistons to compress against the brake rotor, effectively preventing the vehicle from moving. While this definition is uncomplicated, many additional components to this process need to be considered.
The brake pedal does not, of itself, connect directly to all four rotors, but rather, through brake lines, the single force of your foot on the brake pedal is divided to operate the brake system at all four corners of your vehicle. Brake lines are filled with a hydraulic fluid known as brake fluid.
Brake fluid is effective because it is a non-compressible and hygroscopic liquid, which means no gas is released through compression when a person presses down on the brake pedal.
The hydraulic properties of brake fluid allow the pressure created by the brake pedal to be converted through the hydraulic pressure of the brake lines into pressure that compresses the brake rotors.
When the brakes are applied, the brake pistons will compress the brake rotors in order to create friction. The heat generated when applying the brakes will add to the burning of brake fluid during heavy braking. When brake fluid boils or overheats, it will develop air bubbles within the brake system.
The introduction of air creates compressible gas, making it difficult for the brake fluid to move the brake piston through the brake line to slow down the vehicle. As a result, the driver will experience a spongy brake pedal sensation as the brake pressure is applied to the air rather than to the brake pistons.
How To Check Your Brake Fluid – 4 Easy Steps
- To locate the brake master cylinder reservoir, look for it near the back of the engine bay or just next to the firewall. You should also find it directly above the brake pedal on the other side of the firewall. If your vehicle is having problems, you can refer to your owner’s manual to help locate the part.
- Check the level of brake fluid in the master cylinder reservoir.
- Newer cars: The majority of newer vehicles contain a plastic, semi-transparent reservoir for their brake fluid. The full level will be clearly marked on the side of the reservoir. As such, it is possible to check the fluid level without removing the cap from the reservoir.
- Older Cars: Older cars (built before the mid-1980s) contain a metal reservoir that contains a screw on cap secured by a spring clip. Clean the top of the reservoir before removing and inspecting the level of brake fluid. Use a small screwdriver to carefully pry the clip away from the cap and remove the cap to check the fluid level. There should be a mark indicating where to fill the reservoir to.
- Add enough brake fluid to fill to the indicated line (if low).
- NOTE: When your brake fluid level drops, it is an indication that your brake pads have deteriorated and need to be maintained. Get your vehicle to a brake professional as soon as possible.
- IMPORTANT: Never use any type of brake fluid other than that recommended in your owner’s manual. Do not overfill the reservoir. If there are two chambers in your brake fluid reservoir, both chambers must be filled to the indicated level. If your fluid reservoir is at the absolute lowest possible level (or below) it may not be safe to drive your vehicle. Consult with a professional brake technician.
- Replace the cap or cover after the inspection is complete.
6 Common Causes of Brake Fluid Leakage
The following are some of the common sources of brake fluid leaks, and your mechanic can help identify them:
#1. Cracked Brake Master Cylinder Reservoir.
The brake master cylinder reservoir is something that is typically made of plastic. Because of exposure to heat over time, it will become brittle, and as it becomes more brittle, it will crack and allow brake fluid to seep through the back of the engine.
#2. Piston Seal Failure.
Brake components like the master cylinder, disc brake caliper, or drum brake wheel cylinder use Pistons to create the necessary movement to push the brakes away from the wheels (among others).
Because the Pistons are not fixed and the brake fluid is used to move the Pistons, there are seals around the Pistons. These seals do wear out over time, which can create leaks.
#3. Worn Brake Pads and Shoes and Brake Rotors, and Drums.
The brake pads, brake shoes/rotors, brake drums/rotors, and brake drum systems all lose material and break down over time.
When this occurs, the brake caliper pistons and/or drum pistons may become overextended and damage the seals, resulting in leakage of brake fluid.
#4. Broken Brake Lines or Brake Hoses.
Brake lines and brake hoses are designed to withstand almost all of the environmental and driving conditions.
However, over time, brake lines and brake hoses can develop rust, pitting, and/or tears, leading to brake fluid leaks from the brake lines, brake hoses, or brake line fittings.
#5. Loose or Damaged Bleeder Valve.
Every disc brake caliper or drum brake has a bleeder valve or bleeder screw, which is used to evacuate the air that is trapped in the steel brake lines.
If a bleeder valve becomes damaged during the braking process or if a bleeder screw becomes loose, brake fluid might leak.
#6. Faulty ABS Module.
A few components of the brake ABS module use and store brake fluid at extremely high pressures. Over time, the reservoir seals can wear and eventually leak fluid.
If you have a brake fluid leak, you should have the cause identified either by yourself or a qualified mechanic.
How to identify a brake fluid leak?
- To identify a brake fluid leak, check for a small pool of yellowish/brownish fluid under your vehicle. You may also notice that when you press down on the brake pedal, it feels soft or spongy. This may happen if air has been introduced into the braking system through a leak.
- Leaks can come from a number of different components: master cylinder, ABS module, brake line, bleeder valve, rotor/drum, piston(s), and the rest of the braking assembly. By checking which component you see above the leak, you may be able to determine the location of the problem.
- To check for leaks, first lift your car using a jack, block the wheels, and engage the emergency brake. Next, you will want to conduct a thorough visual examination of everything under the vehicle.
- If the leak is slight or difficult to find, you can also put down newspapers under the car (this works best with the vehicle in park and the engine turned off) and then repeatedly press on the brake pedal. Through this process of pressing your brake pedal, you will force brake fluid out of any leaking areas, which then will show up in the newspapers and/or can be seen immediately upon inspection of the leaking component.
- Also, check that the lid of the reservoir is securely fastened. This, too, could potentially be a source of leakage.
Internal leaks may also occur from one part of the braking system to another without producing any external signs (eg, pools of brake fluid underneath your vehicle).
If you are filling the reservoir and the fluid level continues to decrease, and you do not see any obvious signs of leaks, you should not drive your car until you have taken it to your mechanic as quickly as possible.
How to fix a brake fluid leak?
Depending on where the leak comes from, you will either fix it yourself or have to visit a mechanic. When you are unsure which component failed, lack experience or confidence repairing cars, always seek an experienced and qualified mechanic to repair your car.
If you want to repair the car yourself, you will need the following:
- A car jacks
- Wheel blocks
- Parts (dependent on which component you are repairing)
- Car maintenance tools (noted in part-specific repair kits).
How to change brake fluid?
If you want to change the brake fluid yourself, follow these steps:
- Remove the old, dirty fluid from the master cylinder reservoir.
- Use a cheap turkey baster for this.
- Use a lint-free cloth to wipe out the reservoir.
- Do this only if you can get in there.
- Pour new brake fluid into the reservoir just until it reaches the “Full” line, replace the cap on the reservoir.
As you bleed the brakes (which you can find out how to do elsewhere on this site), the new fluid pushes the old fluid out of the system. Continue to bleed the brakes until you see clean, clear fluid exiting the bleeder screw.
How often should I change my brake fluid?
Most brands using Mineral oil-based brake fluid on cars recommend flushing and replacing the brake fluid every three years. In some cases, there is no actual factory guidance for changing brake fluid, as manufacturers say it is “lifetime” for the length of the lease or loan.
However, Haynes recommends replacing brake fluid before the two-year mark and every 30,000 miles. If you drive aggressively or tow, it is suggested that you change your brake fluid at least once per year.
Even though your brake fluid may appear fine, you should replace the fluid on a regular basis due to the moisture content that develops over time at a microscopic level.