View source for Brake fluid

{{Short description|Hydraulic fluid used for braking applications}}
[[File:Delco hydrolic brake fluid, Super 11.JPG|thumb|right|Old brake fluid container]]
[[File:Brake fluid reservoir in Škoda Fabia I.jpg|thumb|right|Brake fluid reservoir in a [[Škoda Fabia]] I]]
[[File:B13 Brake fluid level low.svg|thumb|right|A [[Tell-tale (automotive)|tell-tale]] symbol indicating low brake fluid level]]

'''Brake fluid''' is a type of [[hydraulic fluid]] used in [[hydraulic brake]] and [[Clutch|hydraulic clutch]] applications in [[automobile]]s, [[motorcycle]]s, [[light truck]]s, and some [[bicycle]]s. It is used to transfer force into pressure, and to [[Pascals law#Explanation|amplify]] braking force. It works because [[liquid#Mechanical properties|liquid]]s are not appreciably [[compression (physical)|compressible]].

Most brake fluids used today are [[Glycol ethers|glycol-ether]] based, but [[mineral oil]] (''[[Citroën]]/[[Rolls-Royce Motor Cars|Rolls-Royce]] liquide hydraulique minéral'' ([[Hydropneumatic suspension#LHS versus LHM|LHM]])) and [[silicone]]-based (DOT 5) fluids are also available.<ref>{{cite web|url=http://www.peterverdone.com/archive/files/hydraulic%20system%20theory.pdf|title=Chapter 7 : Basic Hydraulic System Theory|website=Peterverdone.com|access-date=2018-07-06}}</ref>

The origins of modern braking systems date back to 1917, when Scotsman Malcolm Lockheed patented a hydraulic actuated braking system.<ref>{{Cite web|title=How the braking system works|url=https://www.howacarworks.com/basics/how-the-braking-system-works|access-date=2025-01-28|work=www.howacarworks.com}}</ref><ref>{{Cite web|title=Braking 101: The System, Physics and Science of the Motion Management|url=https://dsportmag.com/the-tech/braking-101-the-system-physics-and-science-of-the-motion-management/|access-date=2025-01-28|work=dsportmag.com}}</ref> Initially, vegetable oil was used as a working fluid. But it did not meet the most basic requirements, and in the process of evolution, special brake fluids were created, which consist of a base and a package of additives (thickeners, anti-corrosion additives, colorants).

==Standards==
Most brake fluids are manufactured to meet standards set by international, national, or local organizations or government agencies.

===International===
The [[International Standards Organisation]] has published its standard ISO 4925, defining classes 3, 4, and 5, as well as class 5.1, class 6<ref name="ISO 4925">{{Cite web |url=https://www.iso.org/standard/38513.html |title=ISO 4925:2005 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems |website=www.iso.org}}</ref> and class 7 <ref name="ISO 4925:2020">{{Cite web |url=https://www.iso.org/standard/79370.html |title=ISO 4925:2020 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems |website=www.iso.org}}</ref><ref name="ISO 4925:2020 OBP">{{Cite web |url=https://www.iso.org/obp/ui/#iso:std:iso:4925:ed-3:v1:en |title=Online Browsing Platform ISO 4925:2020 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems |website=www.iso.org}}</ref> reflecting progressively higher performance for brake fluids.

===SAE===
The [[Society of Automotive Engineers]] SAE has published standards J1703, J1704, and J1705, reflecting progressively higher performance for brake fluids. These have counterparts in the international standard, ISO 4925.

===United States===
The Federal Motor Vehicle Safety Standards (FMVSS) under [[Federal Motor Vehicle Safety Standard 116|FMVSS Standard No. 116]]<ref name="FMVSS 116">{{Cite web |url=https://www.govinfo.gov/content/pkg/CFR-2018-title49-vol6/xml/CFR-2018-title49-vol6-sec571-116.xml |title=Code of Federal Regulations, § 571.116 Standard No. 116; Motor vehicle brake fluids.}}</ref> defines grades DOT 3, DOT 4, DOT 5 and DOT 5.1, where DOT refers to the [[U.S. Department of Transportation]]. These are widely used in other countries. Their classifications broadly reflect the SAE's specifications, DOT 3 is equivalent to SAE J1703 and ISO class 3, DOT 4 to SAE J1704 and ISO class 4, etc.<ref name=standards>{{cite web|url=https://wiki.anton-paar.com/en/automotive-brake-fluids/|title=Viscosity of Automotive Brake Fluids|publisher=Anton Paar Wiki|access-date=2018-05-25}}</ref>

All DOT compliant fluids must be colorless or amber, except for DOT 5 silicone, which must be purple. FMVSS Standard No. 116's scope is limited to fluid 'for use'. Brake fluid 'in use', or not labeled DOT compliant, is found in any color.<ref name="FMVSS 116"/>

==== DOT 4 ====
While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 (a temperature upgrade) if the elastomers in the system accept the borate compounds that raise the boiling point,{{citation needed|date=January 2013}} a vehicle that requires DOT 4 might boil the brake fluid if a DOT 3 (a temperature downgrade) is used. Additionally, these [[polyglycol ether|polyglycol-ether]]-based fluids cannot be mixed with DOT 5.0, which is silicone based.

==== DOT 5 ====
DOT 5 is a silicone-based fluid and is separate from the series of DOT 2, 3, 4, 5.1. It is immiscible with water, and with other brake fluids, and must not be mixed with them. Systems can change fluid only after a complete system changeover, such as a total restoration.

It contains at least 70% by weight of a diorgano polysiloxane.<ref>[http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?section=571.116 Standard No. 116; Motor vehicle brake fluids Code of Federal Regulations, Title 49 - Transportation, Chapter V - Part 571 - Federal Motor Vehicle Safety Standards (49CFR571), Subpart B, Sec. 571.116 Standard No. 116; Motor vehicle brake fluids]{{webarchive|url=https://web.archive.org/web/20081208235145/http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?section=571.116 |date=2008-12-08
}}</ref> Unlike polyethylene glycol based fluids, DOT 5 is [[hydrophobic]].<ref>{{cite web |url=https://auto.howstuffworks.com/auto-parts/brakes/brake-parts/types-of-brake-fluid.htm |title=What are the different types of brake fluid? |work=How Stuff Works |date=2008-12-01 |access-date=2018-08-12}}</ref> An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint.<ref>{{Cite web |title=Vehicle Maintenance: Brake Fluid DOT 3, DOT 4, DOT 5.1, and DOT 5 - Valvoline™ Global KSA - EN |url=https://www.valvolineglobal.com/en-ksa/vehicle-maintenance-brake-fluid/ |access-date=2024-06-14 |website=www.valvolineglobal.com}}</ref>

DOT 5 brake fluid is not compatible with [[anti-lock braking system]]s. DOT 5 fluid can aerate when the anti-lock brake system is activated. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air.<ref>{{cite web|url=https://www.freeasestudyguides.com/dot-5-brake-fluid.html|title=DOT 5 Brake Fluid: Not for ABS|website=www.freeasestudyguides.com}}</ref>

==== DOT 5.1 ====
Lack of acceptance of silicone-based fluids led to the development of DOT 5.1, a fluid giving the performance advantages of silicone, whilst retaining some familiarity and compatibility with the glycol ether fluids. DOT 5.1 is the non-silicone version of DOT 5, defined by FMVSS 116 as being less than 70% silicone. Above that threshold makes it DOT 5.

==Characteristics==
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.

===Viscosity===
For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme cold. This is especially important in systems with an [[anti-lock braking system]] (ABS), [[traction control]], and [[stability control]] (ESP), as these systems often use micro-valves and require very rapid activation.<ref>{{cite web|url=http://www.partinfo.co.uk/articles/63|title=Brake Fluid Exchange and Technology|website=Partinfo.co.uk|access-date=2018-05-16}}</ref> DOT 5.1 fluids are specified with low viscosity over a wide range of temperatures, although not all cars fitted with ABS or ESP specify DOT 5.1 brake fluid.<ref>{{cite web|url=https://www.trwaftermarket.com/en/passenger-cars-and-lcv/actuation-systems/brake-fluid/|title=Brake Fluid|website=Trwaftermarket.com|access-date=2018-05-26}}</ref>
For a faster reaction of the ABS and ESP systems, DOT 4 and DOT 5.1 brake fluids exist with low viscosity meeting the maximum 750 {{mm2}}/s viscosity at {{convert|-40|C}} requirement of ISO 4925 class 6.<ref name="ISO 4925"/> These are often named DOT 4+ or Super DOT 4 and DOT 5.1 ESP.

===Boiling point===
Brake fluid is subjected to very high temperatures, especially in the [[wheel cylinder]]s of [[drum brake]]s and [[disk brake]] calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization creates a problem because vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force - so the brakes will fail to stop the vehicle.<ref name=epic>{{cite web|url=http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/|title=DOT Brake Fluid vs. Mineral Oil|website=Epicbleedsolutions.com|access-date=2018-05-25}}</ref>

Quality standards refer to a brake fluid's "dry" and "wet" boiling points. The wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are [[Hygroscopy|hygroscopic]] (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. [[silicone]]/DOT 5 and mineral oil based formulations), are [[hydrophobic]], and can maintain an acceptable boiling point over the fluid's service life.

Silicone based fluid is more compressible than glycol based fluid, leading to brakes with a ''spongy'' feeling.<ref name=epic/> It can potentially suffer phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.{{citation needed|date=May 2015}}

{| class="wikitable"
|+ Characteristics of common braking fluids<ref name="DOT">{{cite web|url=https://www.gpo.gov/fdsys/granule/CFR-2011-title49-vol6/CFR-2011-title49-vol6-sec571-116|title=49 CFR 571.116 - Standard No. 116; Motor vehicle brake fluids.|website=Gpo.gov|access-date=2018-07-06}}</ref><ref name=epic/>
|-
!
! Dry [[boiling point]]
! Wet boiling point{{efn|"Wet" defined as 3.7% water by volume}}
! Viscosity at {{convert|-40|°C|°F}}
! Viscosity at {{convert|100|°C|°F}}
! Primary constituent
|-
| DOT 2 || {{convert|190|°C|°F}} || {{convert|140|°C|°F}} || ? || ? || castor oil/alcohol
|-
| DOT 3 || {{convert|205|°C|°F}} || {{convert|140|°C|°F}} || ≤ 1500 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || [[Glycol ethers|glycol ether]]
|-
| DOT 4 || {{convert|230|°C|°F}} || {{convert|155|°C|°F}} || ≤ 1800 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || glycol ether/[[borate ester]]
|-
| DOT 4+ || {{convert|230|°C|°F}} || {{convert|155|°C|°F}} || ≤ 750 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || glycol ether/[[borate ester]]
|-
| [[Hydropneumatic_suspension#Working_fluid|LHM+]] || {{convert|249|°C|°F}} || {{convert|249|°C|°F}} || ≤ 1200 {{mm2}}/s<ref>{{cite web|url=http://www.viscopedia.com/viscosity-tables/substances/automotive-brake-fluid/|title=Viscosity of Automotive brake fluid – viscosity table and viscosity chart :: Anton Paar Wiki|website=Anton Paar|access-date=2018-07-06}}</ref> || ≥ 6.5 {{mm2}}/s || mineral oil
|-
| DOT 5 || {{convert|260|°C|°F}} || {{convert|180|°C|°F}} || ≤ 900 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || silicone
|-
| DOT 5.1 || {{convert|260|°C|°F}} || {{convert|180|°C|°F}} || ≤ 900 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || glycol ether/borate ester
|-
| DOT 5.1 ESP || {{convert|260|°C|°F}} || {{convert|180|°C|°F}} || ≤ 750 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || glycol ether/borate ester
|-
| ISO 4925 Class 3 || {{convert|205|°C|°F}} || {{convert|140|°C|°F}} || ≤ 1500 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || 
|-
| ISO 4925 Class 4 || {{convert|230|°C|°F}} || {{convert|155|°C|°F}} || ≤ 1500 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || 
|-
| ISO 4925 Class 5-1 || {{convert|260|°C|°F}} || {{convert|180|°C|°F}} || ≤ 900 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || 
|-
| ISO 4925 Class 6 || {{convert|250|°C|°F}} || {{convert|165|°C|°F}} || ≤ 750 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || 
|-
| ISO 4925 Class 7 || {{convert|260|°C|°F}} || {{convert|180|°C|°F}} || ≤ 750 {{mm2}}/s || ≥ 1.5 {{mm2}}/s || 
|}
{{notelist}}

===Corrosion===
Brake fluids must not corrode the metals used inside components such as calipers, wheel cylinders, master cylinders and ABS control valves. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this. Silicone is less corrosive to paintwork than glycol-ether based DOT fluids.<ref name=epic/>

The advantage of the Citroën LHM mineral oil based brake fluid is the absence of corrosion. Seals may wear out at high mileages but otherwise these systems have exceptional longevity. It cannot be used as a substitute without changing seals due to incompatibility with the rubber.<ref>{{cite web|title=AN EXPLANATION OF BRAKE AND CLUTCH FLUIDS|url=http://www.xpowerforums.com/Brake_Fluidsl_FA_Questions.htm|url-status=live|archive-url=https://web.archive.org/web/20081104084534/https://www.xpowerforums.com/Brake_Fluidsl_FA_Questions.htm|archive-date=2008-11-04|access-date=2015-05-26|website=Xpowerforums.com}}</ref>{{User-generated source|date=November 2021}}

===Compressibility===
Brake fluids must maintain a low level of compressibility, even with varying temperatures to accommodate different environmental conditions. This is important to ensure consistent brake pedal feel. As compressibility increases, more brake pedal travel is necessary for the same amount of brake caliper piston force.

== Functions ==
When the driver depresses the brake pedal, pressure is transmitted to the brake master cylinder.<ref>{{Cite web|title=Auto Anatomy: Brake Master Cylinder|url=https://www.dubizzle.com/blog/cars/brake-master-cylinder/|access-date=2025-01-28|work=www.dubizzle.com}}</ref><ref>{{Cite web|title=What is a brake master cylinder or brake pump and what is it used for?|url=https://blog.frenkit.es/en/what-is-a-brake-pump-or-master-cylinder-and-what-is-it-used-for|access-date=2025-01-28|work=blog.frenkit.es}}</ref><ref>{{Cite web|title=What Is Brake Fluid and Everything You Need To Know About It|url=https://nubrakes.com/blog/brake-fluid/|access-date=2025-01-28|work=nubrakes.com}}</ref> The brake cylinder piston pressurizes a system of hydraulic tubes, each of which leads to a different wheel. The brake fluid in the tubes, in turn, pressurizes the brake slave cylinders, which are on each wheel.<ref>{{Cite web|title=How Car Brakes Work|url=https://learndriving.tips/learning-to-drive/car-brakes-tutorial/how-car-brakes-work/|access-date=2025-01-28|work=learndriving.tips}}</ref><ref>{{Cite web|title=How Car Hydraulic Brakes Work and Its Components|url=https://wuling.id/en/blog/autotips/how-car-hydraulic-brakes-work-and-its-components|access-date=2025-01-28|work=wuling.id}}</ref> The slave cylinder pistons press down the brake pads. They encompass and compress the brake disk, and the rotation of the wheels slows down. 

In addition to transmitting pressure, brake fluid also keeps the brake system working optimally. It helps to regulate temperature, ensuring that components are resistant to the heat generated during braking. Proper maintenance of the brake fluid level is critical, as low levels or contaminated fluid can lead to reduced braking performance and, in extreme cases, brake failure.<ref>{{Cite web|title=What You Need to Know about Brake Fluid|url=https://www.garage.co.uk/blog/car-maintenance/what-you-need-to-know-about-brake-fluid|access-date=2025-02-01|work=www.garage.co.uk}}</ref>

Brake fluid is mainly used on brake systems, but is also widely used for hydraulically controlled clutches.<ref>{{Cite web|title=What is Brake Fluid?|url=https://drivesmartwarranty.com/2018/10/26/what-is-brake-fluid/|access-date=2025-01-28|work=drivesmartwarranty.com}}</ref> 

Depending on the application, the fluid is subjected to different pressures: in the case of motorcycles, it has pressure peaks that range from {{cvt|8|to|15|bar|psi}}, while in Formula 1 cars it exceeds {{cvt|75|bar|psi|sigfig=2}}.<ref>{{Cite web|title=Freni sportivi: Tutti i numeri delle frenate|url=https://www.moto.it/news/freni-sportivi-tutti-i-numeri-delle-frenate.html|access-date=2025-01-28|work=www.moto.it}}</ref>

==Service and maintenance==

Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol-based fluid (DOT 3/4/5.1) will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climates, particularly in Russia and Finland.

Brake fluids with different DOT ratings can not always be mixed. DOT 5 should not be mixed with any of the others as mixing of glycol with silicone fluid may cause corrosion because of trapped moisture. DOT 2 should not be mixed with any of the others. DOT 3, DOT 4, and DOT 5.1 are all based on glycol esters and can be mixed, although it is preferable to completely replace existing fluids with fresh to obtain the specified performance.

Brake fluid is toxic<ref name="toxicity">{{cite web|url=http://www.online.petro-canada.ca/datasheets/en_CA/w449.pdf|title=MSDS for DOT 3 brake fluid|website=Online.petro-canada.ca|access-date=2012-06-04}}</ref> and can damage painted surfaces.<ref name="Paint damage by Brake Fluid">{{cite web|url=https://www.totalmotorcycle.com/maintenance/generaltips.htm|title=General Tips|publisher=Total Motorcycle|access-date=2018-05-25}}</ref>

==Components==
{{unreferenced section|date=March 2023}}

=== Castor oil-based (pre-DOT, DOT 2) ===
* [[Castor oil]]
* [[Alcohol (chemistry)|Alcohol]], usually [[butanol]] (red / crimson fluid) or [[ethanol]] (yellow fluid) (methanol)

===Glycol-based (DOT 3, 4, 5.1)===
* [[Alkyl ester]]
* [[Aliphatic]] [[amine]]
* [[Diethylene glycol]]
* [[Diethylene glycol monoethyl ether]]
* [[Diethylene glycol monomethyl ether]]
* [[Dimethyl dipropylene glycol]]
* [[Polyethylene glycol monobutyl ether]]
* [[Polyethylene glycol monomethyl ether]]
* [[Polyethylene oxide]]
* [[Triethylene glycol monobutyl ether]]
* [[Triethylene glycol monoethyl ether]]
* [[Triethylene glycol monomethyl ether]]

===Silicone-based (DOT 5)===
* [[Dioctyl sebacate|Di-2-ethylhexyl sebacate]]
* [[Dimethyl polysiloxane]]
* [[Tributyl phosphate]]

==See also==
* [[Brake fluid pressure sensor]]
* [[Hydropneumatic suspension]]
* [[Pascal's law]]

==References==
{{Reflist}}

== External links ==
{{commons category}}
* [https://web.archive.org/web/20070304074607/http://www.carcare.org/Brakes/brake_fluid.shtml Car care council]
* [https://auto.howstuffworks.com/auto-parts/brakes/brake-parts/types-of-brake-fluid.htm ''How Stuff Works: What are the different types of brake fluid?'']
* [http://www.stoptech.com/tech_info/wp_brakefluid_1a.shtml StopTech: Brake Fluid 1A] {{Webarchive|url=https://web.archive.org/web/20111011083210/http://www.stoptech.com/tech_info/wp_brakefluid_1a.shtml |date=2011-10-11 }}

{{Motor fuel}}
{{Powertrain}}
{{Authority control}}

[[Category:Vehicle braking technologies]]
[[Category:Automotive chemicals]]
[[Category:Hydraulic fluids]]