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Why is it important to do a brake fluid flush?

Brake fluid absorbs and retains water. We cover its characteristics, why periodic flushes prevent harmful moisture buildup, how to perform one, and which tools you'll need.

If you've ever visited an automotive dealership service department, you may have been told that your vehicle needs a brake fluid flush. Or you might have received a coupon mailer offering this service for a discounted price. Maybe you thought it was an attempt on the part of the dealer to separate you from your money for something you don't really need. After all, you have made the effort to learn more about your brakes, and you understand that occasionally, the brake pads and rotors will wear out and need replacing. However, you've never heard of a "brake fluid flush".

Brake Fluid Exchange Banner

Brake fluid flushes, however, are not a case of flim-flam. Take a look at your vehicle owner's manual. Some vehicle manufacturers never recommend a flush, while others recommend it every 2-4 years (varying by vehicle make). The important thing to remember is, brake fluid can absorb moisture, which is bad, and an occasional brake fluid flush can be good preventative maintenance. Within this article, we'll discuss what brake fluid flushes are, why they are important for your vehicle, what the different types of brake fluid are, and more.

Vehicle manufacturers go to great lengths to engineer their vehicles so that they require less maintenance, not more. By doing this, they make overall cost of ownership numbers lower - a statistic that can be very enticing in product advertisements. So you can bet if a particular maintenance service is specified, the engineers that built your vehicle require it for a good reason. In the case of replacing old brake fluid with fresh fluid, it's not just important - it's essential.

What Is A Brake Fluid Flush?

A brake fluid flush refers to the process of replacing all of your old brake fluid with fresh, clean brake fluid. This involves pushing the old fluid out of the entire system as new fluid is added. Creating the pressure necessary to bleed out the old fluid can be done in several ways. Manually, you can either have another person pump the brake pedal or use a hand-pump pressure bleeder tool. Professional shops often rely on power flush machines, and typically charge an hour labor plus the cost of new fluid.

Fluid Bled Out Trough Screw
Bleeder screws are screw valves found on disc brake calipers and drum brake wheel cylinders designed with a hollow center. When the screw is loosened with a wrench, fluid can be bled out directly through it.

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 at each wheel.

Clean Break Fluid

Why Brake Fluid Flushes Are Important

Most brake fluid used in production vehicles today is glycol-based. This type of fluid is "hygroscopic", which means it naturally absorbs moisture that's present in the air at all times. According to engineers, that absorption rate is approximately 1.5 to 3% 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.

Hygroscopic Brake Fluid
As brake fluid naturally absorbs water over time, it becomes contaminated and boils sooner at lower temperatures. This can significantly reduce brake effectiveness - especially if anti-lock brakes are involved.

As brake fluid absorbs more and more water, it will begin to boil at lower and lower temperatures. Remember that brakes work by converting energy - forward motion - into friction, which is dissipated as heat. The heat in the pads and rotors eventually makes its way to the brake fluid in the hoses and lines.

In normal liquid form, brake fluid does not compress - this allows pressure in the system to be transferred consistently and evenly. Boiling is bad because when fluid changes into a gaseous state and aerates into bubbles, it does compress - creating a squishy-feeling pedal and reduced stopping power. In a worse-case scenario, the brake pedal may sink to the floor, giving you no braking at all. Performing a brake fluid flush prevents this gradual decay in the effectiveness of your braking system.

Water Content Chart
This chart shows how the boiling point of brake fluid is affected by percent of water content. Further below in this article is a chart showing the exact boiling points of the various grades of brake fluid.

While water in the brake system is the main reason fluid flushes are performed, the hygroscopic nature of brake fluid actually proves helpful in preventing other types of problems. For example, if brake fluid was not hygroscopic, water (which is heavier by weight) would pool together in pockets and gravitate to lower areas in the system such as the brake calipers. This would create a more serious corrosion problem, and those pockets of water would boil much sooner than when water is dispersed evenly throughout all of the fluid.

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.

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

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-to-replace brake system components is another important reason you should perform brake fluid flushes at the intervals recommended by your vehicle manufacturer.

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.

What Type Of Brake Fluid Should I Use?

Chart Of Exact Boiling Points Of Various Grades Of Brake Fluid
This chart shows the dry and wet boiling points of the different grades of brake fluid. Note that DOT 2 brake fluid is obsolete and no longer available.

Different grades of brake fluid have different dry and wet boiling points. "Dry boiling point" signifies the boiling temperature of brand new, pure fluid with no moisture in it. "Wet boiling point" is the reduced boiling temperature of fluid that's absorbed 3.7% water. Because it only takes about two years for DOT 3, 4, and 5.1 fluids to reach 3.7% water content, doing a brake fluid flush every several years is extremely important.

The U.S. Department of Transportation (DOT) assigns the brake classifications. Each DOT designation signifies fluids have been certified to meet performance standards up to designated temperature points. They are:

DOT3 (Glycol-Based)

Commonly specified by auto manufacturers, this type of brake fluid meets criteria for glycol-based fluids with a wet boiling point of 284 degrees Fahrenheit and a dry boiling point of 401 degrees F.

DOT 4 (Glycol-Based)

Brake Fluid Specification
Most vehicle manufacturers specify using only DOT3 or DOT4 brake fluid.

Like DOT 3 fluid, DOT 4 is also glycol-based. It's suited for heavier-duty use and higher brake temperatures thanks to a higher wet boiling point of 311 degrees Fahrenheit and a dry boiling point of 446 degrees. Vehicle manufacturers of higher-performance models that see aggressive use on the street and/or racetrack often use DOT4.

DOT 5 Synthetic (Silicone-Based)

This is a recently-developed silicone-based synthetic brake fluid that has a boiling point of at least 500 degrees Fahrenheit. Because it does not absorb moisture like DOT 3 & 4 fluids, DOT 5 fluid doesn't need to be changed as a matter of routine maintenance. And like synthetic motor oil, DOT 5 fluid can cost three to five times as much. DOT 5 is NOT suitable for ABS systems because it can aerate when cycled through small orifices. DOT5 can be found in use on military trucks and heavy equipment not equipped with ABS that sit for long periods of time. Additionally, DOT5 will not mix with types 3 or 4 fluid.

According to engineers that work with brake fluids, silicone in DOT5 that's introduced into an older brake system can attach itself to sludge that may already be present due to unrelated component deterioration. This can create gelatinous glop that grows bigger as it draws other contaminants to it. Eventually, metering orifices become clogged and pistons can even stick. Engineers also say that if you've already changed over from 3 or 4 to DOT5, you should switch back. If you've ever gotten silicone on your fingers, you understand how difficult it is to wipe away fully.

DOT 5.1 (Glycol-Based)

This is the non-silicone version of DOT5. Both have the same boiling points, but 5.1 embodies all the other physical characteristics of DOT 3 and 4.

To view brake DOT3, 4, and 5 fluids that we offer, we welcome you to look through the Brake Fluids & Lubricants section of our website.

Tools That Will Help With The Process

Pressure Bleeding vs. Manual Bleeding

"Pressure bleeding" is a process for purging old brake fluid and air out of the system by using a canister with a built-in hand pump. After connecting the pressure bleeder to the brake fluid reservoir, pressure is pumped up by hand. As brake bleeder screws are opened, new fluid from the pressure tank is forced into the lines, and old fluid is forced out. Because pressure is created and held by the pressure bleeder unit, one person is able to do this job easily.

Pressure Brake Fluid Bleeders
One of many pressure brake fluid bleeders we offer.
Manual Bleeding Process

When a pressure bleeder isn't available, "manual bleeding" requires a second person to manually step on the brake pedal at designated intervals. Bleeder screws on the caliper are opened one at a time, starting at the wheel farthest from the master cylinder. After the pedal is fully depressed and brake fluid has flowed out through a tube that's submerged in a container of old fluid, the bleeder screw is closed so that air does not get sucked back into the lines when the pedal rises back up. As this process is repeated at all four wheels, fresh brake fluid should be added to the reservoir so that new fluid continues to come out of the lines.

Fluid Come Out Of Lines

We invite you to look through the Brake Service Tools section of our website. There, you'll find pressure bleeders, adapter fittings for fluid reservoirs, special bleeder screw wrenches, clear plastic tubing for draining old fluid into a bucket, and much more.

At CARiD, we firmly believe in the benefits of educating our customers about the products we sell. We hope this article has been helpful, and we invite you to browse through all the related Helpful Articles on our site. These articles, combined with the replacement brake parts, performance brake parts, and special brake tools we offer, provide you with lots of leeway when deciding to either make the repair on your own or give the job over to a professional. Whichever you choose, you will be the better consumer for it.

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