Are Expensive Brake Fluids Worth the Extra Cost?

Expensive Brake Fluids

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

Share tips, start a discussion or ask one of our experts or other students a question.

Make a comment:
characters remaining

19 Responses to “Are Expensive Brake Fluids Worth the Extra Cost?”

  1. Steven

    Thank you for your informative article.. Langley, BC Canada

  2. Tom Gencarelli

    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 rotted lines!

  3. Michael

    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!

  4. E. Dean Butler

    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.

  5. Stephen Brown

    Your whole story got blown out of the water when ended by saying buy a qualityDOT 4 fluid....what is quality?

  6. Mike Boyajian

    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, Mike Boyajian

  7. Mike Boyajian

    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, Mike Boyajian

  8. Terry

    What about silicone brake fluid?

  9. John

    So the only down side to DOT5 is the cost?

  10. David Dewey

    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.