Understanding the Importance and Frequency of Changing Your Brake Fluid!

(Last Updated On: February 19, 2024)

Brake fluid is the essential component in a braking system, far more critical than brake pads. With no brake fluid, there is nothing to apply the pressure to the brake pads, so it’s essential to ensure the brake fluid is correct and the fluid is in tip-top condition. 

To answer the question, is a brake fluid flush necessary? Yes, it is, but it doesn’t need changing every day or even every month, so there are some conditions around when and why it’s required, which are covered in the article.

What Happens if You Don’t Change Brake Fluid?

Failure to change your brake fluid can lead to various issues due to its hygroscopic nature, which means it absorbs moisture over time. Even though the brake fluid system is sealed, there are instances, for example, when the cap is removed for maintenance, where it can intake outside air (moisture).

When brake fluid absorbs moisture, it faces several problems. Firstly, as the fluid undergoes compression during braking, the absorbed water can reach its boiling point, creating air within the brake system. This air in the brake lines can cause a soft or spongy brake pedal because the compressible air lacks the resistance necessary for effective braking. Usually, the fluid provides the critical resistance, preventing the pedal from dropping to the floor. Experiencing this sudden brake pedal drop can be alarming and unsafe, resulting in a lack of vehicle deceleration.

Secondly, as brake fluid ages, its boiling point decreases. This reduction in the boiling point becomes especially problematic when the brake fluid reaches its wet boiling point, around 311 degrees Fahrenheit/155 degrees Celsius or lower for DOT 4 brake fluid(most common). During aggressive driving or in high-demand situations, the temperature within the brake system can surpass this threshold (commonly reaching 392 degrees Fahrenheit/ 200 degrees Celsius at the brake caliper). At this point, the fluid boils, generating vapor in the form of tiny air bubbles, leading to a similar soft or spongy brake pedal sensation.

To be clear, neglecting to change brake fluid can result in decreased braking efficiency, a spongy brake pedal feel, and potentially dangerous situations where the brakes fail to function optimally due to vapor formation within the system.

How Often Is a Brake Fluid Flush Needed?

All vehicle manufacturers recommend that a complete brake fluid flush is carried out every two years. On most vehicles, it is incorporated as part of the service schedule; however, sometimes, it is an additional service on top of the service cost.

Brake Fluid Inspection and Replacement

Checking the fluid involves removing a small amount of brake fluid from the vehicle; ideally, this should be fluid drained from the caliper, but most people will take some fluid from the reservoir. Place the brake fluid into a cup and insert a fluid boiling point tester to check its resistance to heat. 

New DOT 4 (the industry standard in modern cars) is supplied when new with a dry boiling point of over 446 degrees Fahrenheit (230 degrees Celsius) and a wet boiling point of 311 degrees Fahrenheit (155 degrees Celcius). The dry boiling point means new fluid without any moisture absorbed, and wet is with 3.7% of water absorbed, the standard for fluid between 1 – 2 years old.

A complete brake fluid flush means the entire brake system gets drained and replaced with new fluid, and then the brakes are bled off all air in the braking system. Most garages will have brake fluid pressure bleeders, which replace the fluid with very little air added and keep the brakes under pressure while you bleed the brake system. The cost of having a garage carry out a brake fluid flush means it is usually not worth the time and effort you will put in to do it yourself.

It is possible to drain the brake fluid at home with the right tools. Remember to put the fluid cap back on afterward. However, without experience, getting all remaining air locks from the brake system can be difficult, so it’s sometimes best left to the professionals.

Can You Not Just Add New Brake Fluid?

If the brake fluid level is low, it is safe to top up the brake fluid. However, this would indicate that the brake pads may be worn, so it is wise to inspect them. As the brake pads wear, the fluid travels further, so the fluid level drops. The brake fluid doesn’t burn or wear away; its properties deteriorate.

If the brake fluid is between the minimum and maximum marker, there isn’t any reason to top up the brake fluid because the extra fluid in the reservoir tank will not increase the brake performance of the vehicle or restore the brake fluid’s boiling point. However, if the brake warning light is illuminated and the fluid level is the cause, topping it up will turn the light off. 

brake pedal

Is It Safe to Drive With Brake Fluid Below Its Boiling Point?

Driving with brake fluid below its minimum boiling point is unsafe because the heat at the brake pad could exceed 200 degrees Celsius in an emergency braking situation. If the brake fluid is at the point of boiling at 155 degrees Celsius, the fluid in the brake caliper will boil, which means the brake pedal will travel further, and the braking force required will not be applied (spongy brakes). Essentially, you won’t stop in time.

Frequently Asked Questions

What Happens If Your Brake Fluid Is Dirty?

Driving with dirty brake fluid is just as bad as going with the fluid below its boiling point. Dirty fluid means contaminants, which can aid corrosion and quickly lower the fluid’s boiling point.

Can New Brake Fluid Restore Brake Performance?

If the old brake fluid is below its minimum boiling point, has trapped air, or is contaminated, replacing it will restore any lost brake performance.

Bottom Line

As you can see from this article, a brake fluid flush is essential. It should be carried out every 24 months. If the brake fluid is tested and found to be close to the minimum boiling point before the two years, you should also consider changing it. Driving with brake fluid below the recommended boiling point is unsafe because if the brake fluid boils while braking, the vehicle will not slow down. 

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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