Regular vehicle maintenance is essential for safety and performance, and the critical braking system is one that should never be overlooked. Sure, we all replace the pads and rotors once we start hearing that familiar metal-on-metal grinding noise, and a seized caliper is easy to spot as the vehicle pulls that way or the smell of burning brakes fills the cabin. But something that many drivers forget to check is the brake fluid.
This oft-overlooked liquid is the lifeblood of a car’s braking system, because without brake fluid, the brakes simply don’t work, and the car can’t slow down. Seems pretty important, right? That’s why it’s essential to know how often brake fluid should be changed and how to tell when it’s time to do so.
Understanding Brake Fluid and its Importance
Before getting into any of that, let’s start at the beginning and touch on how brake fluid works and why it’s so important for stopping a car or truck. With my experience as a technician and my Masters degree in mechanical engineering, I could geek out on the fluid dynamics of this stuff all day, but let’s suffice it to say that brake fluid works via hydraulic pressure as an incompressible fluid.
This means that when the driver hits the brakes, the force applied to the brake pedal is transferred through the brake lines to the calipers. As the calipers engage, they press the brake pads against the rotors, creating friction and heat that slows the vehicle. Any compromise in the quality or level of brake fluid can lead to a spongy brake pedal or, in the worst cases, complete brake failure.
There are different types of brake fluids, mainly DOT 3, DOT 4, and DOT 5. DOT 3 and DOT 4 are glycol-based fluids common in most vehicles, whereas DOT 5 is silicone-based and typically used in performance vehicles, and it’s not compatible with vehicles that use anti-lock braking systems (ABS). Knowing the differences between these brake fluids is important to understanding why it must be regularly replaced. You can read more details about DOT standards here in this breakdown from Valvoline.
Why Brake Fluid Needs Replacing
Glycol-based DOT 3 and DOT 4 brake fluids are what’s known as hygroscopic, meaning they absorb moisture from the air over time. This is something that naturally happens through microscopic holes in brake hoses or lines or through poorly sealed reservoirs, and there’s really no stopping it. Over time, tiny water molecules will find their way into the brake fluid, and it’s it’s generally accepted that roughly 1% to 3% of any given brake fluid will be replaced by water every year.
Such a small percentage might not sound like a lot, but as more and more water accumulates in the brake fluid, its boiling point starts coming down. As the brakes are applied repeatedly and heat builds at the pads and rotors, water-laden brake fluid can start to boil. Once that happens, it loses its incompressibility and the hydraulic process can falter, leading to a soft pedal or system failure.
In addition to directly affecting the braking performance, this water buildup can lead to rust and corrosion of the brake lines and seizing of the calipers. These are both far more expensive repairs than replacing the brake fluid, and by the time it gets to this point, the fluid needs to get replaced either way.
Signs it’s Time to Replace Brake Fluid
Replacing the brake fluid before it causes an issue is always going to be ideal, but not everyone knows what to look for. Thankfully, there are two telltale signs that it’s time to flush the brake fluid and the first one we’ve already covered. If the brake pedal starts feeling a little soft, or it’s taking longer to stop than it normally does, that’s a good sign too much water has gotten into the brake fluid and it’s time to get replaced.
Another easy way to tell that something is wrong is by looking at the color of the fluid. In nearly all vehicles, the brake fluid reservoir is located in the engine bay, up near the firewall on the driver’s side. Clean, fresh brake fluid is almost clear in color, perhaps somewhat amber. If the fluid looks dark or murky, that’s an indication it’s full of contaminants and needs to be flushed.
How to Test Brake Fluid
Instead of leaving it up to feeling a soft pedal or noticing the reservoir getting darker, an easy way to tell if it’s time to replace the brake fluid is with a brake fluid tester. These easy-to-use tools can be dipped straight into the brake fluid reservoir – just make sure to clean any dirt and debris out from around the cap before popping it off – and it displays the moisture content with the push of a button.
While there is no hard and fast rule to the exact moisture content that causes issues, the industry standard is that any brake fluid above 4% water should be replaced.
How Often Should Brake Fluid Be Replaced?
There are a few different schools of thought on the interval between brake fluid flushes, with many automakers putting only a mileage limit on it. While working as a technician, I’ve seen owner manuals call for the brake fluid to be changed every 30,000 to 60,000 miles, and while I don’t disagree with this, it’s important not to forget about time.
Recall that most brake fluids are hygroscopic and absorb enough moisture to become roughly 1% to 3% water yearly. This, combined with the fact that any brake fluid with a moisture content of at least 4% should be replaced, means that this service should be done every two years or so, no matter how many miles the vehicle is driven.
Another thing to keep in mind about these timeframes is that they change based on the climate where the car spends most of its time. Due to the hygroscopic nature of brake fluid, humid climates like Mississippi or Florida require brake fluid to be changed more regularly than dry climates like Arizona or Nevada. In high-humidity areas, there is more moisture in the air for the brake fluid to absorb, hitting that 4% threshold even faster.
How Brake Fluid is Replaced
Once it’s time to replace the brake fluid in a vehicle, the process is pretty simple with the right tools. Typically done via flushing, the old fluid is forced out of the system while the new fluid is pumped in. Automotive shops do it this way using a specialized machine that hooks up to the reservoir and, through several hoses, quickly and efficiently swaps out the old with the new. Costing a pretty penny, these machines make a brake fluid flush almost hands off, but they don’t always make sense for a do-it-yourselfer on a budget.
The same process can be mimicked on a smaller scale with something like a vacuum brake bleed kit. All four calipers on a vehicle have a smaller bleeder screw on them, and when pressure is applied to the lines by pushing on the brake pedal and these bleeders are opened, the old fluid can be evacuated. While this happens, new fluid can be forced into the system through the vacuum kit, and this continues until clear brake fluid comes out of each bleeder.
No matter if it’s done at the shop or in the garage, clear brake fluid is happy brake fluid, and this should be done as regular preventative maintenance. Get a tester to make it easy to know when it’s time, or just do a brake system flush every two years, and everything should be A-okay. This is just one aspect of normal brake maintenance that car owners must do, alongside replacing pads and rotors, repairing faulty brake lines, installing new calipers, and anything else that comes up.
Find Everything Needed for a Brake Fluid Flush at CARiD
Making it easy to find everything needed for all kinds of regular maintenance, CARiD provides a shop-by-service feature. When it comes to doing a brake fluid flush, users can simply enter their vehicle year, make, and model, and the site will show compatible brake fluids with different DOT ratings, brake bleeders to help flush the system, and other related items.
For more help finding the right brake fluid and tools needed to properly do a flush, check out the CARiD live chat tool, where experts are standing by 24/7 ready to help.
