Chemistry of Brake Fluids

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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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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
 
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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
 

MolaKule

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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
 
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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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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?
 

MolaKule

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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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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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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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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
 
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Why is brake fluid hygroscopic? It seems counter intuitive. The answer was almost lost to history, and it delayed the introduction of hydraulic brakes almost 20 years. Every time you apply the brakes, the brake fluid in the calipers heats up. Every time the brake fluid heats up, it can absorb more moisture from the atmosphere. The brake fluid surrounding the brake caliper piston is constantly heating up and cooling off. If the that brake fluid was oil based it would absorb some moisture when hot, and drop it out of suspension when cold. The moisture dropped out of suspension would be pure water and it could boil or freeze, with obvious side effects for the braking system. It was learned that brake fluid should absorb water and rapidly diffuse it away from the caliper piston seal area. This is very counter intuitive. Brake fluid is designed to disperse moisture so that water does not build up in the caliper and caliper-piston interface and freeze or boil. The constant hot-cold cycles of an automotive brake caliper cause any fluid to absorb moisture from the atmosphere and the brake fluid absorbs and disperses the water. Most people think moisture enters the brake fluid from the vent on the master cylinder. It does not. Most moisture enters the system through the caliper piston area. If you seal the master cylinder with a rubber diaphragm (some cars and motocycles do this), moisture still builds up in the caliper piston chamber. Aircraft use a different brake fluid because they don't have the constant hot/cold braking cycles that cars go through. Cars originally used the aircraft oil type of brake fluid and they could not make it work. It took early engineers a long time to figure out that water absorption and dispersion were the key properties needed in an automotive hydraulic brake fluid.
 
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Originally Posted By: MolaKule
If you mix a higher boiling point fluid with a lower boiling point fluid, you lower the overall boiling point of the resultant mix.
Not quite correct. The boiling point of the resultant mix in this case will be somewhere between the boiling point of the lowest boiling point fluid and the highest boiling point fluid. It is perfectly fine to use DOT4 where DOT3 is specified, but not the other way around. That's why Valvoline has brake fluid container labeled as DOT3 & DOT4 brake fluid. The reason it is communicated to the public not to mix the two fluids is precisely because manufactures don't want that to be interpreted as DOT3 can be used in place of DOT4.
 
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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?
Seems to me the last container of "4" I bought for my German bike was Castrol and said "3/4" on the label. The bike manual indicates it should be changed one a year. I wouldn't use 3 in a system requiring 4, but with a label which says "3/4" I would use 4 in a 3 rated system. Yes, the boiling point would be lower than pure 4, but still higher than than the pure 3
 
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