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.
Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in vehicles. It is responsible for transferring force into pressure and amplifying braking force. Simply stated, when you apply your foot to the brake pedal, brake fluid transfers this force into pressure to the front and rear brakes and stops the vehicle. It works because liquids are incompressible.
Brake fluid plays a crucial role in the braking process. It is the liquid chemical solution used in hydraulic braking systems to transfer the force of your foot pressing down on the brake pedal into pressure on your vehicle’s brakes.
The act of braking generates extreme heat. This heat causes moisture to condense in the brake hydraulic system. Brake fluid absorbs this moisture and prevents it from boiling and causing brake failure. Additionally, brake fluid lubricates moving parts and prevents corrosion.
Brake fluid must pass testing for the ability to remain fluid at a low temperature and resist boiling at a high temperature. It must be compatible with other brake fluids and parts of the braking system and effectively control the corrosion of your brake system.
Brake fluid works similarly to how your brain fires off electrical impulses to move your muscles. An input is initiated (i.e., you hitting the brake pedal), the brake fluid rushes through the brake system to the calipers and pads, and the car comes to a halt. A mechanic checks the brake fluid level.
When you step on your brake pedal, it forces the small piston(s) inside your brake caliper to compress, squeezing your brake rotors and essentially slowing down your car. Although this is the simplest way to explain the concept, it doesn’t factor in some of the details that allow this to happen.
Because the brake pedal doesn’t just magically connect to all four brake rotors, it relies on the use of brake lines to divert your single force acting on the brake pedal onto the four corners of the car. These lines are filled with a hydraulic fluid we call brake fluid.
Brake fluid works well because it’s a non-compressible, hygroscopic fluid that allows all the energy that your foot releases onto your brake pedal to be converted into braking pressure that squeezes your brake rotors and slows them down. So why is brake fluid so vital, yet often overlooked when shopping for brake rotors?
When you apply your brakes, it causes your brake pistons to compress and clamp the brake rotors. This friction creates heat, lots of it. This means your brake fluid needs to retain its incompressible nature without boiling or evaporating due to the temperature.
What happens when your brake fluid boils or overheats? It creates air bubbles in the system and air is compressibly opposed to brake fluid. So, in turn, when you press down on the brake pedal, instead of clamping the rotors and slowing down the car, the brake fluid compresses the air and the brakes are not doing their job. This also translates to the driver as a mushy or unresponsive feeling brake pedal.
Here are some of the most common culprits of brake fluid leaks that your technician can help you find:
The brake master cylinder reservoir is typically made of plastic and can become brittle from heat exposure. When this happens, it’ll eventually crack, causing brake fluid to seep out and flow down the back of the engine.
Brake components like the master cylinder, disc brake caliper, or drum brake wheel cylinder all function through a piston. The piston is a moving part that’s activated by brake fluid. It has seals that help contain the fluid, and these can be damaged from regular wear and tear, causing a leak.
Brake pads, rotors, brake shoes, and drums can also wear down over time. When this happens, it’s possible for the caliper piston or wheel cylinder piston to become hyperextended, breaking the piston seals and leak fluid.
Brake lines and hoses are designed to withstand most road and weather conditions. But they’re subject to rust, pitting, and tears over time. A broken brake line, a tear in the brake hose, or damaged brake line fittings can all lead to brake fluid leaks.
Each brake caliper or brake drum has a bleeder valve (or bleeder screw) used to “bleed brakes,” which allows air to be expelled from the steel brake lines. If the bleeder valve gets damaged or knocked loose, it can cause brake fluid to leak.
Some parts of the ABS pump in your brakes carry and hold high-pressure brake fluid. Unfortunately, your ABS brake reservoir seals can wear down over time – leading to a brake fluid leak. At this point, you or your mechanic should have figured out the source of your brake fluid leak.
Internal leaks can also occur. In this case, fluid can leak from one component into another without any symptoms such as pools of brake fluid manifesting beneath your car. If your fluid level keeps falling despite top-ups and you cannot see any obvious source or indication of leakage, refrain from driving the vehicle and visit your mechanic as soon as possible.
Fixing a leak is entirely dependent on its source, so if you are unsure which failed component is causing the leak or are not experienced or confident working on your car, always visit a trained mechanic and have them perform the repairs.
To perform fixes yourself, you will need:
If you want to change the brake fluid yourself, follow these steps:
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.
Most manufacturers who use a mineral oil-based brake fluid typically suggest you flush it and refill it with fresh fluid every three years. For some cars, there may not be a factory recommendation, since it will likely be fine for the length of the lease or loan, which the manufacturer calls “lifetime”.
But Haynes still recommends you change your brake fluid every two years or 30,000 miles. If you drive aggressively or tow a trailer, it won’t hurt to do it once a year.
Even if the fluid looks okay, you should still change it periodically. You’re not going to see the moisture in the fluid – it happens on a microscopic level.
