Brake fluid
Brake fluid is a type of
Most brake fluids used today are glycol-ether based, but mineral oil (Citroën/Rolls-Royce liquide hydraulique minéral (LHM)) and silicone-based (DOT 5) fluids are also available.[1]
Standards
Most Brake fluids are manufactured to meet standards set by international, national, or local organizations or government agencies.
International
The
SAE
The
United States
The Federal Motor Vehicle Safety Standards (FMVSS) under
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 any color.[5]
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,[
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.
DOT 5 brake fluid is not compatible with anti-lock braking systems. 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.[9]
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.
Citroën hydropneumatic suspension
In the 1950s, Citroën introduced a
This system was also used on Rolls-Royce and some Maserati models.
Hydragas and Hydrolastic suspension
The fluid was a low viscosity fluid based on diluted alcohol.[11]
- 49% alcohol
- 49% distilled water
- 1% triethanolamine phosphate (surfactant)
- 1% sodium mercaptobenzothiazole (stenching agent)
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
Boiling point
Brake fluid is subjected to very high temperatures, especially in the
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
Silicone based fluid is more compressible than glycol based fluid, leading to brakes with a spongy feeling.[14] 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]
Dry boiling point | Wet boiling point[a] | Viscosity at −40 °C (−40 °F) | Viscosity at 100 °C (212 °F) | Primary constituent | |
---|---|---|---|---|---|
DOT 2 | 190 °C (374 °F) | 140 °C (284 °F) | ? | ? | castor oil/alcohol |
DOT 3 | 205 °C (401 °F) | 140 °C (284 °F) | ≤ 1500 mm2/s | ≥ 1.5 mm2/s | glycol ether |
DOT 4 | 230 °C (446 °F) | 155 °C (311 °F) | ≤ 1800 mm2/s | ≥ 1.5 mm2/s | glycol ether/ borate ester
|
DOT 4+ | 230 °C (446 °F) | 155 °C (311 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s | glycol ether/ borate ester
|
LHM+ | 249 °C (480 °F) | 249 °C (480 °F) | ≤ 1200 mm2/s[16] | ≥ 6.5 mm2/s | mineral oil |
DOT 5 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 900 mm2/s | ≥ 1.5 mm2/s | silicone |
DOT 5.1 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 900 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
DOT 5.1 ESP | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
ISO 4925 Class 3 | 205 °C (401 °F) | 140 °C (284 °F) | ≤ 1500 mm2/s | ≥ 1.5 mm2/s | |
ISO 4925 Class 4 | 230 °C (446 °F) | 155 °C (311 °F) | ≤ 1500 mm2/s | ≥ 1.5 mm2/s | |
ISO 4925 Class 5-1 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 900 mm2/s | ≥ 1.5 mm2/s | |
ISO 4925 Class 6 | 250 °C (482 °F) | 165 °C (329 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s | |
ISO 4925 Class 7 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s |
- ^ "Wet" defined as 3.7% water by volume
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.[14]
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.
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.
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[18] and can damage painted surfaces.[19]
Components
Castor oil-based (pre-DOT, DOT 2)
- Castor oil
- 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)
- Di-2-ethylhexyl sebacate
- Dimethyl polysiloxane
- Tributyl phosphate
See also
References
- ^ "Chapter 7 : Basic Hydraulic System Theory" (PDF). Peterverdone.com. Retrieved 2018-07-06.
- ^ a b "ISO 4925:2005 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems". www.iso.org.
- ^ "ISO 4925:2020 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems". www.iso.org.
- ^ "Online Browsing Platform ISO 4925:2020 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems". www.iso.org.
- ^ a b "Code of Federal Regulations, § 571.116 Standard No. 116; Motor vehicle brake fluids".
- ^ "Viscosity of Automotive Brake Fluids". Anton Paar Wiki. Retrieved 2018-05-25.
- ^ 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 fluidsArchived 2008-12-08 at the Wayback Machine
- ^ "What are the different types of brake fluid?". How Stuff Works. 2008-12-01. Retrieved 2018-08-12.
- ^ "DOT 5 Brake Fluid: Not for ABS". www.freeasestudyguides.com.
- ^ Jackson, Tony; Bardenwerper, Mark L. (March 2016). "Revised Summary of Citroën Hydraulic Fluids". www.citrogsa.com/tony.html.
- ^ "Hydragas suspension technical data". Hydragas Register.
- ^ "Brake Fluid Exchange and Technology". Partinfo.co.uk. Retrieved 2018-05-16.
- ^ "Brake Fluid". Trwaftermarket.com. Retrieved 2018-05-26.
- ^ a b c d "DOT Brake Fluid vs. Mineral Oil". Epicbleedsolutions.com. Retrieved 2018-05-25.
- ^ "49 CFR 571.116 - Standard No. 116; Motor vehicle brake fluids". Gpo.gov. Retrieved 2018-07-06.
- ^ "Viscosity of Automotive brake fluid – viscosity table and viscosity chart :: Anton Paar Wiki". Anton Paar. Retrieved 2018-07-06.
- ^ "AN EXPLANATION OF BRAKE AND CLUTCH FLUIDS". Xpowerforums.com. Archived from the original on 2008-11-04. Retrieved 2015-05-26.
- ^ "MSDS for DOT 3 brake fluid" (PDF). Online.petro-canada.ca. Retrieved 2012-06-04.
- ^ "General Tips". Total Motorcycle. Retrieved 2018-05-25.