Chemistry of Brake Fluids

MolaKule

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by Permission of Author (Molekule) A brake fluid is first and foremost a specialized hydraulic fluid. A hydraulic fluid transmits power from one point in a vehicle (pedal/vacuum boost unit) to another (brake calipers). Hydraulic fluids used in automotive hydraulic brake systems must satisfy a variety of requirements. In manual trasmission equipped cars, this same type of fluid may transmit power from the clutch pedal’s hydraulic cylinder to the clutch actuator cylinder to activate the throw-out bearing, for clutch disengagement during gear changes. In general, the requirements for a brake fluid include chemical and thermal stability, suitable viscosities for the intended use, fluidity over the use-temperature range, low volatility, non-corrosiveness to metals, limited effect on rubber parts and good tolerance for water. Thus, a hydraulic brake fluid to be commercially acceptable is required to meet industry-accepted specifications as well as those established by governmental agencies. Industrial specifications include Society of Automotive Engineers (SAE) specifications such as 7Or1 Artic and 7Or3. Governmental specifications include National Highway Traffic Safety Administration, Department of Transportation, Federal Motor Vehicle Safety Standard 571.116 and 571.116a. Automotive hydraulic brake fluids used today are most often synthetic glycol-base, water-miscible fluids. Brake fluids generally have been blends of several components such as vegetable oils, various alcohol-based fluids, synthetic ester base oils, diluents and one or more oxidation and corrosion inhibitors. Blended hydraulic fluids have contained such lubricants as castor oil, polyoxyalkylene glycols, glycol ricinoleate, and glyceryl ethers of polyoxyalkylene glycols and such diluents as butyl alcohols, amyl alcohols, glycol esters, polyoxyalkylene glycols, monobutyl ethers, ethylene glycol monoalkyl ethers and the like. In addition to the myriad of synthetic base fluids (they can range from 7 to 11), brake fluids have anti-oxidants, corrosion inhibitors, and metal deactivators included. Not all blends have been entirely satisfactory in all cases. Those that are satisfactory with respect to most of the requirements are difficult to prepare, since a blend component that satisfies one requirement may be disadvantageous with respect to another requirement. Thus, a blend component that meets a high boiling point requirement frequently does not meet the low temperature viscosity requirement or a blend component that satisfies the low temperature viscosity requirement may adversely affect rubber parts used in hydraulic systems, e.g., cause swelling, softening, and the like. Brake fluid absorbs moisture form the atmosphere, thus it is hygroscopic. Moisture gradually reduces the boiling point, so the fluid should be changed periodically to remove water and other contaminants and to ensure the continued effectiveness of the braking system. The properties of different types of brake fluids are tested for many different characteristics such as ph value, viscosity, resistance to oxidation, and stability, and graded against compliance standards set by the United States Department of Transportation (DOT) as noted above. Brake fluid DOT specifications: DOT 2 is castor oil based; pretty much obsolete. DOT 3 is composed of various glycol esters and ethers. Boiling point: 284° F (140° C) DOT 4 is also composed of glycol esters and ethers, and boronic compounds. Boiling point: 311° F (155° C) DOT 5 is silicone-based. It is NOT recommended for any vehicle equipped with antilock brakes (ABS). It gives better protection against corrosion, and is more suitable for use in wet driving conditions. Boiling point: 356° F (180° C) DOT 5.1 is a high-boiling point fluid that is suitable for ABS-equipped vehicles. It contains polyalkylene glycol ether, but is more expensive than other brake fluids. Boiling point: 375° F (190.6° C) Even if they have similar base composition, fluids with different DOT ratings must NOT be mixed.
 
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JHZR2

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Originally Posted By: MolaKule
Even if they have similar base composition, fluids with different DOT ratings must NOT be mixed.
Why????? Plenty of folks exchange DOT 3 and 4 fluids, some are even labeled this way!
 
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Originally Posted By: MolaKule
by Permission of Author (Molekule) A brake fluid is first and foremost a specialized hydraulic fluid. A hydraulic fluid transmits power from one point in a vehicle (pedal/vacuum boost unit) to another (brake calipers). Hydraulic fluids used in automotive hydraulic brake systems must satisfy a variety of requirements. In manual trasmission equipped cars, this same type of fluid may transmit power from the clutch pedal’s hydraulic cylinder to the clutch actuator cylinder to activate the throw-out bearing, for clutch disengagement during gear changes. In general, the requirements for a brake fluid include chemical and thermal stability, suitable viscosities for the intended use, fluidity over the use-temperature range, low volatility, non-corrosiveness to metals, limited effect on rubber parts and good tolerance for water. Thus, a hydraulic brake fluid to be commercially acceptable is required to meet industry-accepted specifications as well as those established by governmental agencies. Industrial specifications include Society of Automotive Engineers (SAE) specifications such as 7Or1 Artic and 7Or3. Governmental specifications include National Highway Traffic Safety Administration, Department of Transportation, Federal Motor Vehicle Safety Standard 571.116 and 571.116a. Automotive hydraulic brake fluids used today are most often synthetic glycol-base, water-miscible fluids. Brake fluids generally have been blends of several components such as vegetable oils, various alcohol-based fluids, synthetic ester base oils, diluents and one or more oxidation and corrosion inhibitors. Blended hydraulic fluids have contained such lubricants as castor oil, polyoxyalkylene glycols, glycol ricinoleate, and glyceryl ethers of polyoxyalkylene glycols and such diluents as butyl alcohols, amyl alcohols, glycol esters, polyoxyalkylene glycols, monobutyl ethers, ethylene glycol monoalkyl ethers and the like. In addition to the myriad of synthetic base fluids (they can range from 7 to 11), brake fluids have anti-oxidants, corrosion inhibitors, and metal deactivators included. Not all blends have been entirely satisfactory in all cases. Those that are satisfactory with respect to most of the requirements are difficult to prepare, since a blend component that satisfies one requirement may be disadvantageous with respect to another requirement. Thus, a blend component that meets a high boiling point requirement frequently does not meet the low temperature viscosity requirement or a blend component that satisfies the low temperature viscosity requirement may adversely affect rubber parts used in hydraulic systems, e.g., cause swelling, softening, and the like. Brake fluid absorbs moisture form the atmosphere, thus it is hygroscopic. Moisture gradually reduces the boiling point, so the fluid should be changed periodically to remove water and other contaminants and to ensure the continued effectiveness of the braking system. The properties of different types of brake fluids are tested for many different characteristics such as ph value, viscosity, resistance to oxidation, and stability, and graded against compliance standards set by the United States Department of Transportation (DOT) as noted above. Brake fluid DOT specifications: DOT 2 is castor oil based; pretty much obsolete. DOT 3 is composed of various glycol esters and ethers. Boiling point: 284° F (140° C) DOT 4 is also composed of glycol esters and ethers, and boronic compounds. Boiling point: 311° F (155° C) DOT 5 is silicone-based. It is NOT recommended for any vehicle equipped with antilock brakes (ABS). It gives better protection against corrosion, and is more suitable for use in wet driving conditions. Boiling point: 356° F (180° C) DOT 5.1 is a high-boiling point fluid that is suitable for ABS-equipped vehicles. It contains polyalkylene glycol ether, but is more expensive than other brake fluids. Boiling point: 375° F (190.6° C) Even if they have similar base composition, fluids with different DOT ratings must NOT be mixed.
Where's that "like" button? laugh In practice, I bet there's been a whole heckuva lot of mixing of DOT3 and DOT4 fluids... in fact I've probably done it myself when DOT4 first came out. DOT5 and higher- not so much since its always been a "special" fluid and you generally had to know something about its properties before you even considered buying it.
 

MolaKule

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Quote:
Why????? Plenty of folks exchange DOT 3 and 4 fluids, some are even labeled this way!
The main difference between DOT 3, 4, and 5.1 fluids are the inclusion of higher boiling point fluids in the higher number designation fluid, and the addition of boron compounds in DOT 4 and 5.1 fluids. If you mix a higher boiling point fluid with a lower boiling point fluid, you lower the overall boiling point of the resultant mix. Hence the recommendation NOT to mix. Since Brake Fluid is a safety item, I would not mix, regardless of what people do or what the label states in this regard.
 
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Originally Posted By: JHZR2
Originally Posted By: MolaKule
Even if they have similar base composition, fluids with different DOT ratings must NOT be mixed.
Why????? Plenty of folks exchange DOT 3 and 4 fluids, some are even labeled this way!
Was going to ask the same. People mix/add/top off Dot 4 fluids to dot 3 vehicles all the time?
 
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It isn't a mix but when people change brake fluid from DOT-3 to Dot-4(or vice versa) with a good flush from bleeder valves, there is some old DOT-3 fluid left in the system. What will happen ?
 
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Originally Posted By: Johnny248
Why can't Dot 5 be used in ABS systems?
I was going to ask the same question. My Guess is, Dot 5 has a tendency to hold micro air bubbles (which is why it is sometimes claimed to give a 'spongy' brake pedal) I guess, they could like micro 'springs' in the fluid and defeat the rapid ABS on-off pluses. OK, I know I did not express that well :-(
 

MolaKule

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Originally Posted By: expat
Originally Posted By: Johnny248
Why can't Dot 5 be used in ABS systems?
I was going to ask the same question. My Guess is, Dot 5 has a tendency to hold micro air bubbles (which is why it is sometimes claimed to give a 'spongy' brake pedal) I guess, they could like micro 'springs' in the fluid and defeat the rapid ABS on-off pluses. OK, I know I did not express that well :-(
Dot 5 brake fluid is based on silicones (actually siloxanes), is therefore not compatible with glycol-based brake fluids. Dot 5 fluid is more compressible due to aeration and foaming under normal braking conditions. An ABS system uses a timed pulse for fluid flow metering and does not look at pressure/volume for feedback, so Dot 5 fluid is not a good candidate for ABS. The DOT 3, 4, and 5.1 glycol-based fluids however are not compressible and thus are more suitable for ABS systems.
 
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MolaKule

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Originally Posted By: HTSS_TR
It isn't a mix but when people change brake fluid from DOT-3 to Dot-4(or vice versa) with a good flush from bleeder valves, there is some old DOT-3 fluid left in the system. What will happen ?
Dot 5.1 can be used to replace Dot 4 and 3, and Dot 4 can be used to replace Dot 3. There is always some old fluid left in the system somewhere, but as much old fluid should be removed as possible. The formulations between each DOT brake fluid category is slightly different.
Quote:
The main difference between DOT 3, 4, and 5.1 fluids are the inclusion of higher boiling point fluids in the higher number designation fluid, and the addition of boron compounds in DOT 4 and 5.1 fluids. If you mix a higher boiling point fluid with a lower boiling point fluid, you lower the overall boiling point of the resultant mix. Hence the recommendation NOT to mix.
 
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Tay

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i read that DOT 5.1 is much more hygroscopic then DOT 4 and DOT 4 more then DOT 3. is there any truth to this?
 
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Thanks for that. I, too, had heard such a thing mentioned. Of course, it's never something from a company that makes the stuff or in a sourced article. It's just mentioned in passing in some general tech article.
 

MolaKule

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Originally Posted By: Garak
Thanks for that. I, too, had heard such a thing mentioned. Of course, it's never something from a company that makes the stuff or in a sourced article. It's just mentioned in passing in some general tech article.
Exactly, inferential information from the Internet is dangerous.
 
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I blame the German automakers. wink In North America, where everyone specifies DOT 3, few (if any) manufacturers call for a brake fluid replacement interval. The Germans, who tend to call for DOT 4, often do call for a replacement interval. Therefore, there must be something wrong with the DOT 4 fluid, rather than just a different take on maintenance, of course.
 
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Thanks for the article Molakule!
Originally Posted By: tc1446
Some brand name cans say "Dot 3/4". I read this as it can be used either way or mixed. right or wrong?
Probably, just like motor oils. I personally wouldn't mix exclusively separate chemistries. Mixing different ethers/blends could, as Mola pointed out, change the boiling point of the resultant fluid, and this is where one enters into unknown-spec territory. Compatability-wise, if you had to do it, you could. shrug
 

MolaKule

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Originally Posted By: tc1446
Some brand name cans say "Dot 3/4". I read this as it can be used either way or mixed. right or wrong?
What the label is saying is that it is a DOT 4 fluid with a higher boiling point that can also be used in DOT 3 applications. Again gentlemen and ladies, let us not compare brake fluid to motor oil or other fluids. It is a very specialized hydraulic fluid and is hygroscopic which means it can absorb moisture, which means you need to change it every three years or less. Let me stress again, Brake fluid is a SAFETY ITEM.
 
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