Brake Upgrades…What works, and what doesn’t. Part 2, fluids

In part 1 of this series, we looked at brake rotors, and what upgrades and features are available in the aftermarket.  More importantly, we learned that some upgrades are better than others, and that it is important to consider upgrading rotors only as part of a well thought out, complete package. In this installment, we will look at another key component, brake fluid.

Most modern automotive braking systems use glycol-ester based fluids, differentiated primarily by their DOT ratings.  These ratings determine several characteristics of the fluid, the most recognized of which is their MINIMUM boiling point:

* DOT 5.1 is fluid which meets the standard of DOT 5, but contains less than 70% silicone.  DOT 5 fluid is by standard more than 70% silicone, is not for use in ABS vehicles, and does not mix with non-silicone fluids. As such, it doesn’t really have a place in a discussion about modern street car brakes.

The term ‘boiling point’ is just that, the point when the fluid will begin to boil.  Brakes work by turning mechanical energy into heat energy, and some of this heat energy is transmitted to the fluid. If the brake fluid temperature exceeds the boiling point, gas bubbles will form, and since gas compresses much easier than liquid, braking power will be diminished, contributing to an overall loss of braking ability known as brake fade.

Differences in wet and dry boiling points come from the moisture absorbing (hygroscopic) nature of the fluid, which lowers its boiling point.  Moisture builds up over time, and is unavoidable, as it enters the system through microscopic holes in seals, hoses, and fittings.  Despite the drop in boiling point, the hygroscopic nature of brake fluid is actually desirable.  Huh?  Yep, absorption of system water is actually a good thing, and here’s why.  Since it is impossible to keep water out of the braking system, absorbing it into the fluid, and therefore disbursing it through the entire system is far better than having it remain in one area, where it can cause corrosion, boil, or freeze. Eventually though, the fluid will absorb too much water, along with dirt and other contaminants, and so it must be changed periodically.  On a regular street vehicle, the change interval is somewhere on the order of every 1-2 years.  On a car that is tracked regularly, this interval could be monthly or even weekly. On a race car, it is not uncommon to change the fluid before every race. As we learned with rotors, braking consistency ranks a close second to braking efficiency when it comes to performance driving.  With the large differences seen between wet and dry fluid performance, it is easy to see how overall system performance could be affected as the fluid gets ‘wetter’.  Dirty fluid compounds the problem by increasing wear in the system.

So what fluid should you be using?  Since you are reading this, I’m going to assume you have an interest in high performance braking.  As such, you might be thinking that DOT 5.1 seems like the obvious answer, and it might be, but it might not be, either.  Each of these fluids has different chemistries, which lead to different properties, and these can vary from manufacturer to manufacturer.

The federal standard covering brake fluid is FMVSS116, and in addition to boiling point, each DOT fluid must meet many other requirements, including viscosity, pH, oxidation resistance, and a host of others.  One thing that is not specified amongst the non-silicone fluids is chemical composition.  They don’t even have to be glycol-ester based; that is just what the industry has settled on.  Each manufacturer is free to use different base stocks and additives in their fluid, and as long as they meet the standard, can label it as DOT approved, and sell it.  Other than boiling point, the other standard which separates the DOT standards is viscosity at -40C: DOT 4 is allowed to be slightly thicker than DOT 3, while DOT 5.1 must be considerably thinner than either.  So, if a manufacturer compounds a fluid meeting the boiling point requirements of 5.1, but only meeting the viscosity standard of DOT 4, they must call it DOT 4.  And remember, the boiling points are minimums, so it is not uncommon to find a DOT 4 fluid with higher boiling points than a DOT 5.1 fluid.

Here is a quick look at three different DOT 4 fluids, and a DOT 5.1 fluid:

About the only thing the Napa fluid has going for it is availability, as you can get it, or its equivalent, at just about any auto parts store.  You’ll need to find a higher end speed shop, or turn to the internet, for the others.  But since brake fluid changes are seldom done without some level of planning, availability becomes a non-factor.  Amongst the others, ATE is quite good, and actually comes in two colors, so that when flushing the system, you can tell when all of the old fluid is out.  The SRF is the hands down winner in performance, and is what we use in the race cars and in our customers’ track cars.  The downside is obviously the price.  The Motul is an interesting fluid, in that its DOT 5.1 classification means that it will have a lower viscosity, which could improve the action of ABS systems, but at the expense of a lower boiling point.

If the fluid in your system is more than two years old, any of these would be an improvement.

So it seems, as with most other areas of automotive performance, there are many options and variables when it comes to brake fluid.  This is why we recommend a well thought out approach to any project, vs. just buying what is cheapest, most expensive, or hottest on the forums.  In future articles, we will look at pads and lines, and maybe even look at some aspects of the hydraulic system design, such as brake pressure and bias. As always, feel free to contact one of the experts here at Blackdog, if you have any questions.

One last point worth making concerning glycol based brake fluids is that they will severely damage automotive paints and some plastics.  Extreme care needs to be taken to keep fluid from coming into contact with anything other than the insides of the braking system.