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#3076040 - 07/25/13 12:52 PM Chemistry of Brake Fluids
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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.


Edited by MolaKule (07/25/13 12:53 PM)
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#3076051 - 07/25/13 01:05 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
JHZR2 Offline



Registered: 12/14/02
Posts: 35439
Loc: New Jersey
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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#3076052 - 07/25/13 01:05 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
440Magnum Offline


Registered: 02/01/09
Posts: 6818
Loc: Texas
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.
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#3076066 - 07/25/13 01:19 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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.


Edited by MolaKule (07/25/13 01:23 PM)
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#3076247 - 07/25/13 04:45 PM Re: Chemistry of Brake Fluids [Re: JHZR2]
Johnny248 Offline


Registered: 07/19/09
Posts: 1979
Loc: Detroit, MI
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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#3076248 - 07/25/13 04:46 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
Johnny248 Offline


Registered: 07/19/09
Posts: 1979
Loc: Detroit, MI
Why can't Dot 5 be used in ABS systems?
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#3076264 - 07/25/13 05:12 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
HTSS_TR Online   content


Registered: 04/17/06
Posts: 14347
Loc: Lake Forest, CA
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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#3076275 - 07/25/13 05:27 PM Re: Chemistry of Brake Fluids [Re: Johnny248]
expat Offline


Registered: 05/12/09
Posts: 4201
Loc: Canada
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 :-(

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#3076384 - 07/25/13 08:03 PM Re: Chemistry of Brake Fluids [Re: expat]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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.


Edited by MolaKule (07/25/13 08:11 PM)
_________________________
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#3076391 - 07/25/13 08:08 PM Re: Chemistry of Brake Fluids [Re: HTSS_TR]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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.


Edited by MolaKule (07/25/13 08:10 PM)
_________________________
I went shopping at the SuperMarket on an empty stomach. I am now the proud owner of Isle #4. smile

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#3077508 - 07/27/13 02:02 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Garak Offline


Registered: 12/05/09
Posts: 12441
Loc: Regina, Saskatchewan, Canada
Great information, Mola! This should be a sticky somewhere.
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2008 Infiniti G37 coupe - Mobil Delvac 1 ESP 5w-40, Hastings LF113
1984 F-150 4.9L six - Quaker State GB 10w-30, Wix 51515

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#3079454 - 07/29/13 12:55 AM Re: Chemistry of Brake Fluids [Re: Garak]
Jakebrake Offline


Registered: 09/10/02
Posts: 280
Loc: Fallon, Nevada
Originally Posted By: Garak
Great information, Mola! This should be a sticky somewhere.

X2

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#3084986 - 08/03/13 10:31 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
Tay Offline


Registered: 01/12/11
Posts: 62
Loc: Asia
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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#3085871 - 08/04/13 07:50 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
Looking at the chemistry I don't see any one fluid more hygroscopic than the other.
_________________________
I went shopping at the SuperMarket on an empty stomach. I am now the proud owner of Isle #4. smile

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#3086127 - 08/05/13 05:21 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Garak Offline


Registered: 12/05/09
Posts: 12441
Loc: Regina, Saskatchewan, Canada
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.
_________________________
Plain, simple Garak.

2008 Infiniti G37 coupe - Mobil Delvac 1 ESP 5w-40, Hastings LF113
1984 F-150 4.9L six - Quaker State GB 10w-30, Wix 51515

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#3086379 - 08/05/13 10:35 AM Re: Chemistry of Brake Fluids [Re: Garak]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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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#3087147 - 08/06/13 01:48 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Garak Offline


Registered: 12/05/09
Posts: 12441
Loc: Regina, Saskatchewan, Canada
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.
_________________________
Plain, simple Garak.

2008 Infiniti G37 coupe - Mobil Delvac 1 ESP 5w-40, Hastings LF113
1984 F-150 4.9L six - Quaker State GB 10w-30, Wix 51515

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#3087252 - 08/06/13 07:48 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
tc1446 Offline


Registered: 12/10/10
Posts: 547
Loc: NC
Some brand name cans say "Dot 3/4". I read this as it can be used either way or mixed. right or wrong?
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#3087726 - 08/06/13 03:52 PM Re: Chemistry of Brake Fluids [Re: tc1446]
jrustles Offline


Registered: 02/24/13
Posts: 2035
Loc: Ontario, Canada
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
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#3087920 - 08/06/13 06:43 PM Re: Chemistry of Brake Fluids [Re: tc1446]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
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.
_________________________
I went shopping at the SuperMarket on an empty stomach. I am now the proud owner of Isle #4. smile

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#3157913 - 10/16/13 04:35 PM Re: Chemistry of Brake Fluids [Re: Tay]
gpshumway Offline


Registered: 06/18/08
Posts: 415
Loc: Minneapolis, MN
Originally Posted By: Tay
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?


Not necessarily more total absorption for DOT 4, but they tend to absorb moisture at a higher rate. German cars take DOT 4 fluid, but require replacement every 2 years (in general), Asian cars take DOT 3 fluid, but only require replacement every 3 years.

Stoptech has a good white paper on this:
http://www.stoptech.com/technical-support/technical-white-papers/brake-fluid

Originally Posted By: Stoptech
The real differentiating factor is that DOT 4 fluid should be changed more often than a DOT 3 fluid, because of the effects and rates of water absorption.
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#3162635 - 10/21/13 01:10 PM Re: Chemistry of Brake Fluids [Re: JHZR2]
miro Offline


Registered: 05/05/13
Posts: 75
Loc: the Netherlands
Thanks for the huge contribution in this forum.
I really like to read your articles. I like the way how you explain- with simple words expunging such complex area like modern fluid chemistry


Edited by miro (10/21/13 01:11 PM)

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#3352601 - 04/25/14 08:52 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Oldmoparguy1 Offline


Registered: 01/21/05
Posts: 4263
Loc: Charlotte, NC
I've always wondered, why did the auto industry settle on glycol type brake fluid? The aircraft industry uses a petroleum based fluid, at least in my experience. Looks like ATF.

Wayne
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#3353381 - 04/25/14 09:46 PM Re: Chemistry of Brake Fluids [Re: MolaKule]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
I think it was due to five things:

1. Brake fluid system pressure

2. brake line materials

3. seal materials

4. compressibility of comparative fluids

5. flammability and vapors


Edited by MolaKule (04/25/14 09:47 PM)
_________________________
I went shopping at the SuperMarket on an empty stomach. I am now the proud owner of Isle #4. smile

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#3634288 - 02/15/15 03:35 AM Re: Chemistry of Brake Fluids [Re: Johnny248]
Woox300sx Offline


Registered: 02/14/15
Posts: 1
Loc: Down Under
Originally Posted By: Johnny248
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?


Dot 4 brake fluids contain borate esters to increase the boiling point. However, borate esters can cause rubbers seals made of SBR (styrene-butadiene rubber) in the master cylinder to swell. This can cause tears in the rubber as the piston moves back and forth.

Castrol LMA DOT 4 markets their product as suitable for DOT 3 systems since it only causes 1-2% increase in rubber swell compared to some other DOT 4 fomulations (eg. Valvoline) which cause up to a 16% increase in rubber swelling. (FMVSS 116 standard).

If your vehicle manufacturer recommends only DOT 3 fluid (Eg. Toyotas), stick to the manufacturer recommendation to avoid any damage to the master cylinder rubber seals.

Bleed your brake fluid regularly (at least every 2 years - every year if you live in a humid climate) & don't drive your car like an idiot. Then you won't notice the different boiling points between the DOT fluid standards.

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#3661555 - 03/11/15 04:24 PM Re: Chemistry of Brake Fluids [Re: Woox300sx]
badtlc Offline


Registered: 06/08/06
Posts: 4379
Loc: KC
Originally Posted By: Woox300sx


Dot 4 brake fluids contain borate esters to increase the boiling point. However, borate esters can cause rubbers seals made of SBR (styrene-butadiene rubber) in the master cylinder to swell. This can cause tears in the rubber as the piston moves back and forth.

Castrol LMA DOT 4 markets their product as suitable for DOT 3 systems since it only causes 1-2% increase in rubber swell compared to some other DOT 4 fomulations (eg. Valvoline) which cause up to a 16% increase in rubber swelling. (FMVSS 116 standard).

If your vehicle manufacturer recommends only DOT 3 fluid (Eg. Toyotas), stick to the manufacturer recommendation to avoid any damage to the master cylinder rubber seals.

Bleed your brake fluid regularly (at least every 2 years - every year if you live in a humid climate) & don't drive your car like an idiot. Then you won't notice the different boiling points between the DOT fluid standards.


Do you have a link to that valvoline data?
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#3662770 - 03/12/15 07:18 PM Re: Chemistry of Brake Fluids [Re: badtlc]
MolaKule Offline


Registered: 06/05/02
Posts: 15205
Loc: Iowegia - USA
Originally Posted By: badtlc
Originally Posted By: Woox300sx


Dot 4 brake fluids contain borate esters to increase the boiling point. However, borate esters can cause rubbers seals made of SBR (styrene-butadiene rubber) in the master cylinder to swell. This can cause tears in the rubber as the piston moves back and forth.

Castrol LMA DOT 4 markets their product as suitable for DOT 3 systems since it only causes 1-2% increase in rubber swell compared to some other DOT 4 fomulations (eg. Valvoline) which cause up to a 16% increase in rubber swelling. (FMVSS 116 standard).

If your vehicle manufacturer recommends only DOT 3 fluid (Eg. Toyotas), stick to the manufacturer recommendation to avoid any damage to the master cylinder rubber seals.

Bleed your brake fluid regularly (at least every 2 years - every year if you live in a humid climate) & don't drive your car like an idiot. Then you won't notice the different boiling points between the DOT fluid standards.


Do you have a link to that valvoline data?


I'd like to see that as well.

There are other esters in the mix, other than just borates or borate ester.
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#3665609 - 03/15/15 10:19 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Claud Offline


Registered: 02/11/14
Posts: 103
Loc: Margate England
I wonder what the change interval recommended for Cuban brake fluid was?.
It seems it was almost unobtainable under Fidel Castros regime, so the Cubans made a concoction of shampoo, rum. and sugar that worked. Apparently white sugar worked best foe Soviet built vehicles, brown sugar for American iron.
Necessity is indeed the mother of invention.

Claud.

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#3695738 - 04/11/15 10:26 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Jimbo Offline


Registered: 02/13/03
Posts: 1350
Loc: California, USA
Originally Posted By: Oldmoparguy1
I've always wondered, why did the auto industry settle on glycol type brake fluid? The aircraft industry uses a petroleum based fluid, at least in my experience. Looks like ATF.



Originally Posted By: MolaKule
I think it was due to five things:

1. Brake fluid system pressure

2. brake line materials

3. seal materials

4. compressibility of comparative fluids

5. flammability and vapors


I don't know what automotive engineer's requirements were, most of the above are probably true. Here is the aviation perspective.

The most common aviation brake/hydraulic fluid is MIL-PRF-5606H, mineral based, such as Aeroshell 41. Non-flammable phosphate ester Skydrol fluid is a whole different animal, toxic and requiring different seal materials. So, seal materials didn't dictate the type of fluid used, but the other way around.

It looks like the most important characteristic is low temperature viscosity.
5606 calls for a maximum kinematic viscosity of 600 @ -40C and 2500 @ -54C.
At high altitude the airframe can cold soak and then descend for a landing and brake application without adequate time for the fluid to significantly warm.

Hygroscopic brake fluids would not be a good idea because moisture absorbed on the ground can form ice crystals at extremely low temperatures at altitude.

5606 seal swell is 19-30% (L rubber). I don't know what "L" rubber is exactly.

5606 flashpoint is >82C vs >135C for DOT 3, not a huge difference.

What is interesting is that Chevron ATF is repackaged and sold as an aviation brake fluid for some light plane braking systems that are not legally required to use 5606.

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69 Karmann Ghia, Delo 400 15W-40
74 VW Thing, Delo 400 15W-40
05 Tundra V8, Chevron Supreme 5W-30
10 Honda Fit, Chevron Supreme 5W-20

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