Let’s be clear up front—if your brake fluid has not been changed in over a year, you are past due to replace it. Brake fluid is hygroscopic—it naturally absorbs water from humidity present in the air. Therefore, it requires regular maintenance.
This article is meant to help you make an informed choice when buying brake fluid at the parts store.
A lot of technical innovations in automotive chemicals have been linked to better performance, increased engine or component life or a less toxic, more earth-friendly disposal for spent fluids. Brake fluid—the hydraulic liquid that is used to “push” your brakes when pressure is applied—has similarly gone through evolutionary development.
Very much like engine oil or power steering fluid, there are dozens of brake fluid choices at your local parts store. They carry everything from the store brand DOT 3 fluid costing a buck, to very high-end synthetic fluids that may cost twenty dollars per can. Keep in mind, the same rules that apply to selecting engine oil apply to brake fluid: the more expensive option is not always the best option.
Some of the brake fluids available for racing applications might perform well when used in those conditions but might not be right for the street.
So, what is the difference between DOT 3, DOT 4, DOT 5, and most recently DOT 5.1?
The US Department of Transportation (that’s what the DOT stands for) established specifications defining a number of PROPERTIES to which brake fluid must adhere without defining chemical composition. Those specifications relate to boiling point of the fluid (both dry and wet), how viscous (flowable) the fluid is, and stability of the fluid at high temperatures among other properties. Succeeding generations of DOT fluid standards have raised the minimum boiling point. By the way, “dry” designates new, unused brake fluid with 0% water. “Wet” fluid is measured for the boiling point standard has absorbed up to 3.7% water.
While we make efforts to keep brake systems impervious and “dry” over time, even a buttoned-up brake system with tight seals and new lines absorbs some moisture. The key here is what happens to that moisture after it enters the system.
Absorption of water from humidity over time lowers the boiling point, making it more likely the fluid will boil. Picture summer driving in the city. The constant stop/start in traffic gets your calipers extra hot. This allows the fluid to boil as it reaches hot calipers. Gas or vapor formed when liquid reaches its boiling point allows the fluid to “compress,” making for longer travel when you apply the brakes. Typically, people describe this as having a “soft” brake pedal. In the worst of these situations, you may need to pump the brakes to have them take action.
The most common brake fluids—DOT 3 fluids are primarily glycol ether; DOT 4 fluids are also glycol-ether based, but have borate esters added to increase the boiling points. DOT 5 fluid was manufactured using silicone, which does not absorb water.
The point behind creating a silicone-based fluid was to avoid water absorption. Unfortunately, water still gets into the brake system, pooling or puddling rather than being absorbed into the fluid. That leads to corrosion in the system.
Most folks know they aren’t supposed to top off DOT 3 or 4 brake fluids with DOT 5, but don’t know why. The answer goes back to the chemistry. Combining even trace amounts of a glycol-based brake fluid with DOT 5 can cause the two incompatible fluids to gel, resulting in poor braking. Converting to DOT 5 also requires thorough flushing and removing ALL traces of the old fluid to avoid seal damage.
Let’s review: Brakes get wicked hot especially under extreme conditions. When the calipers (and the fluid reaching them) get hot, that fluid can—and will—boil. Boiling produces gas, which is more compressible than the fluid leading to soft, spongy brake pedal feel and a longer travel time when applying the brakes. As water enters the over time, hygroscopic brake fluid begins absorbing water from the atmosphere. Brake fluid containing water boils with less heat.
Our recommendation to most car enthusiasts is to go with a high-quality DOT 4 fluid. The higher-quality fluids offer a chemical makeup that is more resistant to moisture and contain the proper rust inhibitors we need for our classic cars.
Now, to wrap up, let’s talk frequency of changing out your old brake fluid. Put it on your once yearly maintenance list and you are likely pretty well covered. (A cautionary note here—you should change DOT 4 fluid more frequently than a DOT 3 fluid, because water will be absorbed more quickly in the DOT 4 fluid.)
Article Courtesy of Master Power Brakes
Thank you for your informative article.. Langley, BC Canada
Dot 5 (Silicone Based Fluid) does NOT Damage Paint like Dot 3 or 4. Dot 3 or 4 can damage paint in about 30 mins if not cleaned properly. If you have a high end or custom paint job and you fear that a line breaking could cause an expensive paint repair or for that matter not being able to match the existing paint you should seriously consider DOT 5. If you are installing a completely new brake system then I would say use DOT 5 from the start. Side Note: If you are installing a complete new system use Stainless Steel brake lines…no rotted lines!
This subject is hard to understand without graphics. X/Y chart of fluid v time would show where NEW fluid’s boiling temp starts out but diminishes. Calling it a contradiction is unfair. I had a family member in a funeral convoy lose brakes after riding the pedal in a slow moving procession. The fluid boiled!
I have never seen evidence that brake fluid should be replaced anything like yearly. I worry about the credibility of anyone who claims yearly replacement is important.
Your whole story got blown out of the water when ended by saying buy a qualityDOT 4 fluid….what is quality?
I put in comments about the use of silicone brake fluid. A note said”comments awaiting moderation,” which I didn’t understand, and then my whole commentary disappeared. Please advise,
Silicone fluid Is OK in a vintage or classic car to protect painted surfaces and avoid absorption of moisture, but DO NOT use silicone fluid in a race car. Silicone fluid has a higher compressibility than glycol-based fluids and with heavy usage will result in a “long” brake pedal. I know this from having used silicone fluid in my Elva MK7 years ago and lost a race, thankfully without going off or having damage. In those years, I led a design team for a US Navy spec’d shipboard, ammo-carrying forklift. Silicone fluid was called for, so I could get all I wanted for my Elva. Later, after the race back at home, I checked the silicone specs and learned of its high compressibility. I cleaned and flushed the brake system and refilled it with Ford Motor Co. heavy duty truck brake fluid. I never had another brake problem.
Interestingly, years ago, Lynne St. James, who was sponsored by Dow-Corning for a sedan race (I believe) used silicone fluid in her car and it suffered in the race due to a “long” pedal. At that time their team thought the cause was incompatibility due to fluid mixing, but it was surely due to compressibility.
Regards and stay well,
What about silicone brake fluid?
Hi Terry. Silicone Brake fluid is DOT5 fluid.
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So the only down side to DOT5 is the cost?
Hi John. DOT 5, is more expensive, yes… Also it does compress some leading to a softer pedal feel, water in the system tends to separate and pool as water is attracted to water, which can also result in a spongy pedal and pockets of corrosion. and it does not mix with DOT 3 or 4.
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DOT 4 fluid was originally Girling fluid, used in British brake systems. The British systems were not compatible with DOT3 fluid, which would degrade the seals and lead to catastrophic brake failure. I think this should have been mentioned in your article, especially since this group is oriented toward “Classic” cars. Note that this is a modern usage of “Classic” as it used to be that Classic Cars were only those recognized by the Classic Automobile Club of America, but is now used to describe most any collectable automobile.
Hello David. Thank you for your feedback. I have forwarded your comment to the proper department. We value your opinion and it will help with the development of our online streaming community. We will continue to listen and work hard for your complete satisfaction.
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Article was flat out horrible. Does not mention 5.1 at all, just contradicts itself and leaves open ended statements all over. Why is water supposedly absorbed more quickly in DOT 4 than DOT 3 when they are pretty much the same?
Hello Stan. Thank you for your feedback. I have forwarded your comment to the proper department. We value your opinion and it will help with the development of our online streaming community. We will continue to listen and work hard for your complete satisfaction.
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Hi Stan. Yes, you are correct but keep in mind 5.1 was not released at the time this article was written and most “Classic Cars” are designed to use DOT 3 or 4. Sorry you didn’t like the article but we can differ in our opinions.
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Excellent article. I’m new to the classic car world witj my 1966 GTO. so am all about learning everything I can.
Hi Jim, if your restoring or updating your 66′ GTO go with Dot 5. You MUST completely flush out the old fluid before your convert. Like is mentioned in this article DOT5 does not absorb moisture and for cars like these which sit, are not daily drivers for the most part and have limited use this is the only fluid I would put into these cars, moisture and rust and your worst enemy. I would suggest to fully purge the system is go with all new lines, clean out all the wheel cylinders, and master cylinder if you want to do this the right way and possibly preserve the master cylinder and wheel cylinders if original parts are still part of the braking system. Otherwise you can get all the necessary after market parts to overhaul your system and start from scratch. For me it is keeping my cars original but for many this isn’t a concern unless your going for a true concourse look and originality. Good Luck
please review the “wrap up” paragraph to the “our recommendation” paragraph. They are contradictory with regards to the 3 and 4 fluid and water absorption. Also the DOT fluid type is not stated in the last sentence leaving questions unanswered.
I really enjoy the articles otherwise!
DOT 4 fluid should be changed yearly. Boils at a higher temp but absorbs water faster. Used for the track that is a bad combination since you are looking for better performance. I have boiled DOT 3 …what came with my car on the first track day. First thing I did on car was replace the fluid.
I noticed you stated that city driving could result in higher temperatures, which is totally logical. I am a rural carrier for the Postal Service and am required to use my own personal vehicle. On my route I drive 38 miles total, with 22 miles being in one subdivision. I start and stop at 700 mail boxes within a 4 hour time period. I drive a Toyota RAV4 hybrid and get great mileage. I do know that brake pads and rotors last much longer on hybrid models than they do on non-hybrid models even under these extreme conditions. Should we change our fluid more often under these conditions?
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I’d like to know more regarding the comment about gel forming from small amounts of glycol based fluid mixed with silicone fluid. I have never experienced this – I have used DOT 5 for over 20 years in 8 different vehicles. In one case I flushed the system with DOT 5 and 8 years later found traces of DOT 3 when I went to rebuild a wheel cylinder. Its very easy to see – they separate very well. In other fluid bled from systems I have found traces of glycol fluid as well – always sinks to bottom of the container but always very fluid – never gel. I thought it was because the boiling point of either was compromised in the mixture.
I’ve owned cars for over 50 years with millions of miles logged. Some with over 200k on them and not once have I changed brake fluid and not once have I had a brake failure due to old fluid. Come on “once a year change”
I agree, lasts a lot longer than a year, this article didnt tell me anything of any use.
Maybe you live in the high desert, like I do, and do not have high ambient humidity that can more readily add moisture to the brake fluid. Perhaps you never heat up your brake fluid to levels that can cause it to break down. Maybe you have just been lucky! I would have to guess, though, the performance of your fluid has diminished over time (so gradually, it would not be noticeable) and a flush and refill would result in improvement you could actually feel. Yearly change? Maybe not, if you are not subjecting your cars to harsh conditions and usage.
Have owned 35 cars,never had a break problem, and never change all the fluid, just the stuff in the master cylinder,unless I’m doing caliper work or major break repair,looks like the makers of break fluid got to you folks !!
I have been driving cars for 46 years and have never changed the brake fluid on a vehicle. Many of my vehicles had or have 200,000+ miles and have never had a problem. So to say that a person is only waiting for something to happen is full of it!!
In my experience it is more critical when a vehicle is stored or not driven for a period of time and the fluid sits in the system with no movement. I have experienced calipers seizing more than once during storage due to moisture in glycol fluid. Interestingly this stopped when I switched to DOT 5.
Just to add – even if driven daily the boiling point will lower due to moisture. I did experience glycol fluid boiling once – not fun. But if you never get into a critical situation you might never experience it. In my case the fluid was heated due to driving with the parking brake on …. doh!!!
The article doesn’t actually say whether or not if Expensive Brake Fluids are Worth The Extra Cost! also do you have to change all the fluid or just a couple of inches at each wheel?
Hi, Richard. In this article the fine folks at Master Power Brakes are actually cautioning against using the higher priced DOT5 fluids. Instead recommending the lower priced DOT4 fluid but also cautioned the need to change fluids more often as DOT4 does absorb water faster than the old standby DOT3.
When changing/flushing brake fluid in your car the intent should be to replace ALL of the fluid. While perhaps not 100% feasible, like changing your engine oil, there is always a little of the dirty oil still in the motor.
I generally pull the majority of the fluid out of my master cylinder using my vacuum brake bleeder, then replace that with new fluid, next I bleed the lines at each wheel until I am satisfied enough fluid has been expelled to fill the lines with all new fluid.
LARRY’S COMMENT IS RIGHT,THE ARTICLE CONTRADICTS ITSELF ABOUT DOT-4 FLUID AN WATER ABSORPTION,OR DO THEY MEAN DOT-3 FLUID?
Sorry for the confusion. Both DOT 3 and DOT 4 are hygroscopic fluids that absorb water.
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The article contradicts itself.
The higher quality fluids offer a chemical makeup that is more resistant to moisture and contain the proper rust inhibitors we need for our classic cars.
(A cautionary note here- you should change DOT 4 fluid more frequently than a DOT 3 fluid, because water will be absorbed more quickly in the DOT fluid.)