Brake Fluids

OZbrick: This article is a modified version of an article directed towards Triumph owners. Most of the info is generic...

by Kenneth Streeter, with input from Mike Burdick, Shane Ingate, Chris Kantarjiev, a "Skinned Knuckles" article, and various other sources 

The discussion of whether to use DOT3, DOT4, DOT5, or the new DOT5.1 brake fluids is a common topic. The information provided herein should
help you to decide which of these brake fluids are best for you and your car. I would point out that I am not an "expert" on the topic, but have collected the
experiences of many other enthusiasts, as well as opinions of professional auto restorers. I have tempered my findings with my own experiences and

I would also take this opportunity to point out that the type of brake fluid used in your car is far less important, from a safety standpoint, than a properly
functioning braking system. If you are working on your own brakes, be extremely careful, don't skimp on poor components, and bleed the brake system very
carefully and thoroughly.


     DOT3 brake fluid is the "conventional" brake fluid used in most vehicles. One of the most familiar brands is "Prestone."



DOT4 brake fluid is the brake fluid suggested for use in late model cars. The most familiar brand is "Castrol GT-LMA"




     DOT5 brake fluid is also known as "silicone" brake fluid.




     DOT5.1 is a relatively new brake fluid that is causing no end of confusion amongst mechanics. The DOT could avoid a lot of confusion by giving this new
     fluid a different designation. The 5.1 designation could lead one to believe that it's a modification of silicone-based DOT 5 brake fluid. Calling it 4.1 or 6
     might have been more appropriate since it's a glycol-based fluid like the DOT 3 and 4 types, not silicone-based like DOT 5 fluid. (In fact, Spectro is
     marketing a similar new fluid which they are calling Supreme DOT 4, which seems less confusing.)

     As far as the basic behavior of 5.1 fluids, they are much like "high performance" DOT4 fluids, rather than traditional DOT5 brake fluids.



General Recommendations:

   1.If you have a brake system that doesn't leak or show any other signs of failure, but has old seals in it, don't change fluid types as a result of reading this
     article. If it isn't broken, don't "fix" it -- you may simply break it instead!
   2.Flushing of the brake system every couple years to remove any absorbed or collected water is probably a good idea to prevent corrosion, regardless of the
     type of brake fluid used.
   3.DOT3 is dangerous to use in cars with natural rubber seals, and thus should not be used in such cars, except as a temporary "quick fix to get me
     home" solution. (If this is used as a "get-me-home" solution, bleed the system as soon as possible, and be prepared to replace all your seals.)
   4.DOT3 is an adequate brake fluid for use in later cars, although it is rarely preferred. My recommendation would be to simply not use it.
   5.DOT4 fluid, for a slight increase in cost, will give significantly increased resistance to moisture absorption, thus decreasing the likelihood of corrosion
     compared to DOT3.
   6.DOT4 fluid has a higher boiling point than DOT3, making it preferable for high performance uses such as racing, autocross, or excessive use of the brakes
     in mountainous areas. For even greater braking performance, consider going to DOT5.1 or a high-performance version of DOT4 fluid.
   7.DOT5 is a good choice for the weekend driver/show car. It doesn't absorb water and it doesn't eat paint. One caveat is that because it doesn't absorb
     water, water that gets in the system will tend to collect at low points. In this scenario, it would actually be promoting corrosion!
   8.DOT5 is probably not the thing to use in your race car although it is rated to stand up to the heat generated during racing conditions. The reason for this
     recommendation is the difficult bleeding mentioned above.
   9.When changing from one fluid type to another, as a minimum, bleed all of the old fluid out of the system completely. For best results, all the seals in the
     system should be replaced.
  10.As always, your experiences may vary.


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