Brake Fluid:
The Life Blood of Your Brakes

At first I had started writing this and got a bit carried away on a dissertation of hydraulic systems. Then I realized it was not needed to explain the details surrounding the hydraulic fluids themselves in this case brake fluid. That said I will likely post that info as a link in the reference section of the web page and link it at the top of the brake section. Since I have thus saved you from reading all of this I will just right into fluids.

Brake fluid has one job: transmit pressure from the master cylinder to the slave cylinder. There are several characteristics of brake fluid which must be considered however. 1) it should be relatively thin so that its viscosity does not noticeably effect the response of the brake system, 2) it must be able to withstand heat without boiling. We aren’t so much concerned about #1 as I have never really heard about one fluid being especially better than the other but I could be wrong. But realize that it is a consideration the chemists must make when trying to design fluids which try to optimize #2.

Now it still isn’t all that simple. First of all any fluid that is made needs to be compatible with existing brake systems (seals and soft lines) or it will be relatively useless. Along those same lines it needs to play well with others and mix with other fluids commonly used. If not, then even if it will work with the existing materials on the market it will require a complete flushing of the system when it is used. Yet another constraint put on the chemists.

For reasons mostly rooted in the above paragraph, current brake fluids are glycol based and they are hydroscopic, meaning they absorb water. These fluids are rated DOT 3 and DOT4. So what you say. Well the problem is when brake fluid absorbs water, or gets dirty for that matter, its boiling temperature drops. If you remember from the main section where I talked about fluid fade, this is a bad thing. Boiling fluid means your go no stoppie.

There are fluids out there, namely DOT 5 silicone based, which have a higher boiling point to start with and are non-hydroscopic, meaning they do not absorb water. This is actually bad because any water that does get in your system will now boil at 212F rather than simply depressing the boiling point of the brake fluid. In addition, Silicone based fluids are not compatible with DOT 3/4 fluids. It mixes like oil and water and will actually coagulate; not a good thing. To successfully change to DOT 5 requires a complete flush of the system which is rarely successful, so you end up pretty much taking your entire system apart and cleaning it. Who uses DOT 5: show cars worried about paint protection, Harley straight from the factory again for paint protection, and the military. To top it all off silicone based fluids are twice as compressible as glycol based fluids and results in a mushier pedal with less feedback.

All that being the case not all DOT 3/4 fluids are created equal. Some absorb water more readily, some have a higher dry boiling point, a higher wet boiling point, and the difference between wet and dry can vary. And depending on your application, and frequency of fluid changes you can get away with different fluids.

The federal specifications for fluids is as follows:

I’m showing you this because it is important to understand what goes into determining the spec of a fluid. AFCO's 570º brake fluid is a DOT 3 type fluid. However, it has a dry boiling point that is 52º higher than DOT 5.1 specifications, 124º higher than DOT 4 specifications and 169º higher than DOT 3 specifications. AFCO's 570º fluid meets or exceeds all DOT 3, 4, and 5.1 lubrication, corrosion protection and viscosity specifications.
 
 

AFCO's 570º racing fluid meets but does not exceed federal standards for wet boiling point specification; therefore, its classification is DOT 3. Because AFCO's 570º fluid is intended for use in racing type brake systems that undergo frequent fluid changes, exceeding federal standards for wet boiling points is of little concern. Racing brake fluids always exceeds the DOT specifications for dry boiling points. Wet boiling points generally remain the same. This is why you can’t just look at the spec and must look at the actual boiling points.

DOT 5.1 fluid is also glycol based and is a low viscosity spec used in the early days of ABS. While it is compatible with DOT 3 and DOT 4 don’t be fooled into thinking it is automatically better because it is a higher number. Check the wet and dry boiling points. Depending on the brand some DOT 4 fluids might be better.

For those of you who love knowing little useless facts: wet boiling point is measured at a 3% concentration of water.

That’s about it really. So what fluids to use? The common street and light duty fluid recommend by most people used to be Castrol GTLMA. It is easy to find at most parts stores and is very reasonably priced. Recently though, Valvoline has come out with a "synthetic" fluid which is DOT 3/4 spece’d and has a higher dry and wet boiling point. Again it is easy to find and priced on par with GTLMA. For most of you this is what you should buy. Two points to remember: once you have opened a container of brake fluid use it and discard it; it will absorb moisture sitting on your shelf over time and be worthless. The second is brake fluid eats paint and FAST. Use rags to prevent getting it on your paint in the first place and have them ready to wipe it off fast if you make a mistake. I typically wrap a rag around the reservoir to prevent any spillage from dripping down.

For those of you looking to go to the next level who are having boiling problems there is Ford Blue which comes highly recommended by many racers and is available at most Ford Dealers. I know it is a bit more than the other fluids I mentioned but not so much so that it is prohibitive. Another is Motul 600c brake fluid. I’m sure many real speed shops sell it but if you can’t find it local Road Race Engineering sells it for $9/0.5L bottle. I would suggest buying two for the first time to ensure you totally flush your system and subsequent times you can get away with 1 for maintenance flushes. I have not used this or even heard much about it but if RRE uses it in their rally cars flying down roads with cliffs on one side and trees on the other I have no fear it is good stuff. There is also the AFCO fluid mentioned above. I only just found this stuff while searching the net for information on fluid so I have no information on it beyond what their web site says. Beyond this there are many fluids you can find at some of the more reputable speed shops (notice I didn’t say Rice shops) around your town. Look for ads that mention Porches, BMW’s, road racing, rally, etc. The yellow pages and a phone are a great place to start.
 
 

My final plug is for Speed Bleeders. I won’t bore you with the details as you can get them from the web sight. I will just say that they are worth it period end dot. If you don’t want to order them go to any store you know that carries Russell products (Pep Boys is one but not all carry them) and look for the speed bleeders there. Most people won’t have a damn clue what you are asking about so just tell them it is a Russell part and looks just like the bleed fittings on calipers but you are NOT looking for a brake bleeder kit. Trust me on this one :)

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