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Wildthings
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Joined: March 13, 2005
Posts: 50348
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 Posted: Fri Oct 31, 2014 10:22 am Post subject: |
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For what it is worth, there is often little or no difference between DOT 3 and DOT 4 brake fluids in the same brand. If a company handles both then they have to make two different products that probably only cost them a penny or two difference per quart, so it is easier to just make one product and label and price it as the consumer demands.
Years back, I went through a bunch of MSDS and found that sometimes a companies DOT 3 fluid had a higher reported boiling point than their DOT 4 fluid. I am not claiming here that MSDS are highly accurate, as I do not believe they are. |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21519
Location: Oklahoma City
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| Posted: Fri Oct 31, 2014 11:13 am Post subject: |
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| Wildthings wrote: |
For what it is worth, there is often little or no difference between DOT 3 and DOT 4 brake fluids in the same brand. If a company handles both then they have to make two different products that probably only cost them a penny or two difference per quart, so it is easier to just make one product and label and price it as the consumer demands.
Years back, I went through a bunch of MSDS and found that sometimes a companies DOT 3 fluid had a higher reported boiling point than their DOT 4 fluid. I am not claiming here that MSDS are highly accurate, as I do not believe they are. |
 .....yes.....many times this is correct!
Typically its uncommon that a brake fluid will actually list a higher actual boiling point of a 3 over a 4.....but because there are minimums ....but no maximums (a range)....sometimes you CAN find a DOT 3 that is in the top of its range....and a DOT 4 in the bottom of its range and have the DOT 3 be a higher boiling point.
Some of the high end brands (Pentosin, ATE etc.) have variations like this.
However....the boiling point does not tell the total tale. The viscosity changes (lowers) as temperature increases.
Typically....DOT 4 fluid and not re-badged DOT 3...which i would think would be rare because its illegal ....if it is DOT 4....contains Borate esters which increase the boiling point AND increase viscosity stability at all temperatures.
Years ago...before virtually anything we drive...think just after WW-2...when many brake seals could be made of Buna-N or EPR rubber.....those seals could not live well with the Borate Ester of the DOT 4 when it came on line.
So...you till have DOT 3 fluids.
Many companies are phasing DOT 3 out slowly...because even reproduction seals for antique cars are made of modern EPDM....so there is no reason to keep a fluid around whose baseline chemistry is virtually identical to DOT 4....but is inferior in moisture absorption, viscosity stability and wet and dry boiling point (in general).
But another detail which supports changing fluid regularly. Even though DOT 4 is superior to DOT 3 in most respects.....it has a shorter life 
Once it starts absorbing moisture...its wet boiling point drops faster.
Nice short article http://www.stoptech.com/technical-support/technical-white-papers/brake-fluid
I always ask.....because so many people say just use whats dime-store cheap.....and i wonder how many of these people....actually know what brake fade due to fluid boiling feels like.
many people just consider that its an old car with poor brakes and they get hot and give out because they are an old design. Many times thats not the case.
Better brake fluids are dirt cheap performance and insurance...considering you wont use more than a liter and the difference is under $20 for a flush and fill....and it may make the difference between blowing a stop sign at the end of a long hill some day....or not. Ray |
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