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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: 15046
Loc: Iowegia
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: 12043
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: 523
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: 15046
Loc: Iowegia
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: 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: 74
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: 4171
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: 15046
Loc: Iowegia
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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