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When Is It Time To Replace My Car’s Brake Fluid?

Between repair shops and carmakers, a lot of conflicting information exists about how often brake fluid should be replaced, or whether it should be changed at all. We shed some light on the topic and provide our recommendations.

When it comes automotive maintenance items, changing a vehicle's brake fluid is probably not at the top of anyone's mind. But it should be, because there are important reasons for doing it. In this article, we'll look at conflicting information you may see regarding how often you should change brake fluid. We'll point out why it's important and make our best recommendations for you. In the end, it's a decision you'll make, but you'll be more knowledgeable about why.

If you've completely overlooked brake fluid service up until this point, that's understandable. Often, it's tucked in as a footnote on the specified maintenance pages of your owner's manual. And if your vehicle rolled off the assembly line in the last several years, it might not even be mentioned at all. On this note, we'll mention a trend we've noticed lately among automakers regarding brake fluid.

Automakers Have Changed Their Tune On Brake Fluid Maintenance

We'll start by looking back to when anti-lock brake systems (ABS) became standard equipment on vehicles across all price ranges during the 1990s. At that time, many automakers began specifying a firm maintenance requirement that the brake system be flushed with new fluid every 2 years - regardless of miles. This was mostly the norm for awhile - going unquestioned because engineers who initially spent years testing and developing your vehicle had the best idea of what maintenance items needed to be done. Anything specified was done so for a good reason.

The time interval was (and is) the important factor because over time, no matter how much or how little you drive, brake fluid naturally absorbs moisture from the outside air. We'll get into how bad this is further in the article, but, in short, it can greatly reduce braking ability and cause corrosion of sensitive brake and ABS components.

Fast forward to today where other items such as synthetic motor oil and coolant have become popular with automakers for two simple reasons - they perform better, and last longer. Lasting longer means maintenance intervals can be stretched out further with more miles or time between them. This creates a lower overall cost of ownership, which has become more enticing to potential buyers watching every penny - especially as car loans stretching out to 6, 7, even 8 years are becoming more common.

While synthetic (DOT 5 silicone) brake fluid does exist, it's not recommended for use in anti-lock brake systems. In fact, brake fluid that's available for mainstream automotive use today is still the same glycol-based product that it's always been. It's still "hygroscopic" - naturally absorbing about 1.5 to 3% water a year in areas of normal atmospheric pressures. In humid climates, that rate can climb even higher. Moisture will always find its way into the lines through microscopic pores in brake hoses, seams, joints, and seals - there's simply no way to avoid it.

Some of the same automakers that used to adamantly recommend flushes every 2 years have switched to 3-year intervals, while others make no mention of it at all anymore. It's possible they have concluded that brake fluid flushes are not needed every 2 years. But when you consider that water content level of brake fluid can reach 6% to 8% in the third year of service, such reasoning seems hollow.

However, we suspect the stretch is due to the fact that many automakers now provide free maintenance for the first 2 to 3 services. Stretching a brake fluid change out to the 3-year mark puts it just outside of the covered period, so the customer now pays for it instead. And if there's no free maintenance plan, stretching brake fluid to 3 years results in lower cost of ownership numbers.

Considering all that was once taught on the subject, we're inclined to recommend replacing brake fluid every 2 years if you live in a climate when any portion of the year has high humidity levels. If you're in a dry desert climate, we suggest checking the water content of your brake fluid with a brake fluid tester (see below) at 2 years, and flush it if the level is 4% or higher.

Is There An Easy Way To Tell If I Need A Brake Fluid Flush?

Typical OEM Brake Fluid Tester
Brake fluid testers such as this one sense and report the percentage of moisture in brake fluid.

A brake fluid tester is a useful tool for monitoring the condition of your brake fluid, because it senses and reports the percentage of moisture that's been absorbed. We've got several proven choices from OTC, Mountain Tools, Powerbuilt, and Shark Industries just to name a few. Once you've got a tester, you won't have to wonder if a dealership or other shop recommending a brake flush is trying to sell you something you don't need.

Why Clean Brake Fluid Is Important

As brake fluid takes on water, its boiling point begins to drop to a lower and lower temperature. This can be problematic, because brake fluid in the hoses and lines can get pretty hot due to heat radiating from pads and rotors.

Heavily Contaminated Boiled Brake Fluid
As brake fluid naturally absorbs water over time, it becomes contaminated and boils sooner at lower temperatures.

In normal liquid form, brake fluid does not compress. For this reason, pressure is transferred consistently and evenly throughout the entire system. But if brake fluid boils, it changes into a gaseous state and aerates into bubbles which do compress - leaving a squishy-feeling pedal and reduced stopping power. Performing regular brake fluid maintenance prevents this gradual decay in the effectiveness of your braking system.

It's also important to note that corrosion of metal brake lines and moving parts such as calipers and master cylinder pistons can and will occur eventually if moisture in old brake fluid builds to a significant point. These metal parts can actually corrode from the inside out. A brake line, which fails, can lead to partial or complete brake failure.

Gritty corrosion on parts surfaces causes seals to wear and leak, and it causes calipers to bind up to the point where they no longer apply or release the brakes effectively. Preventing corrosion of expensive brake system components is another important reason you should perform brake fluid flushes regularly.

You can also reference our related article entitled "Why is it important to do a brake fluid flush?"

Opportune Times To Replace Brake Fluid

While we are big believers in the regularly-scheduled brake flush interval, we also recognize that it is an additional expense. With that in mind, we are also always looking for any opportunity to save a few bucks, and one way to do that is by combining related service and repair work.

Disc brake pads and rotors require replacement frequently enough. While their replacement does not normally entail opening the hydraulic system, at least the vehicle is up on a lift with its wheels and tires off. Asking the shop to perform the brake fluid flush at that time will save some pennies.

Should you need caliper replacement, the system needs to be bled anyway. Caliper work is a much more appropriate time to have the entire brake system flushed and refilled, at minimal increased cost alongside the caliper job.

About The Brake Fluid Service Itself

To thoroughly remove all old brake fluid, a proper "flush" is required. During a flush, pressure is created within the system that pushes out the old fluid as new fluid takes its place. This pressure can be created manually by using a second person to pump the brake pedal, or by using a hand-pump pressure bleeder tool. Professional shops often rely on power flush machines (such as the Symtech Brake Flush Assistant), and will charge for their labor plus the cost of new fluid.

Bleeder Kit With Tubes Rubber Elbows And Bottles
When a partner is not available, bleeder kits with tubes, rubber elbows, and bottles make the task easy.
Brake Fluid Pressure Bleeder
Brake fluid pressure bleeders such as this one make it easy for one person to flush old brake fluid out of the lines.

Pressure bleeders, whether power or manual, fit over the brake fluid reservoir and are secured in place with the use of adapter pieces. As pressure and new fluid are added, old brake fluid is removed by loosening and tightening bleeder screws at the brake caliper of each wheel.

Symtech Power Operated Brake Fluid Flush Machine
Power operated brake fluid flush machines such as this one from Symtech allow a single user to flush and bleed an entire brake system in less than 15 minutes.

We hope we've helped you gain an understanding of the characteristics of brake fluid, and how important it is to replace it at regular intervals. We invite you to check out our related article Special Tools Used In Brake Service, which provides a quick 5-minute education on hand tools and other items that prove themselves useful during this procedure and other repairs. And if you have any questions on items we sell, give us a call! Product specialists are here seven days a week.

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