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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: 17332
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: 18829
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.
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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: 706
Loc: Lexington, 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: 17332
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.
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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: 417
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: 91
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 Online   content


Registered: 01/21/05
Posts: 5135
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: 17332
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)
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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: 4982
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: 17332
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: 135
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: 1351
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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#3712304 - 04/27/15 05:28 AM Re: Chemistry of Brake Fluids [Re: MolaKule]
Ducked Offline


Registered: 10/25/12
Posts: 1001
Loc: Taiwan
Originally Posted By: MolaKule
by Permission of Author (Molekule)


DOT 5 is silicone-based... It gives better protection against corrosion, and is more suitable for use in wet driving conditions.



I've seen this plausibly (I thought) disputed.

IIRC the argument was that, although the fluid doesn't absorb water, this can mean that any water entering the system can exist as separate droplets, which can lead to localised pitting corrosion.

Presumably if there is any free water it'll boil at roughly 100C, assuming the system is at atmospheric pressure


Edited by Ducked (04/27/15 05:28 AM)

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