® Brake Fluid
Revised 2016

Purple silicone brake fluid. The color warns and reminds the user: do not put regular glycol fluid in here! It's also kind of pretty.

DoT 3 versus DoT 4, versus DoT 5. Glycol versus silicone. Change intervals, bleeding, certain brake fluids to avoid, politics, potholes, rhetoric, opinion, bumps in the night and and sun spots. Whew! Can all this be simplified?

Air? From Where?
Let's begin at the beginning. A brake circuit is not a sealed system. A physics principle known as Boyle's Law in a round-about way establishes that a fluid cannot leave a container unless air can follow it. If the vent in your gas tank plugs up, for example, fuel will eventually stop flowing. Similarly, a brake system is, like a fuel tank, vented to atmosphere, usually at its highest point, the reservoir. (Ever notice that little notch?) In addition, most brake hoses are rubber, which breathes, so again air can enter. Your brake system has continuous access to air.

The Problem with Moisture
But that means it also has continuous access to moisture, for air naturally has moisture in it, to varying degrees. Therefore, all brake systems have moisture in them. All of them. All the time. And this is unavoidable. There is an inherent difficulty with moisture inclusion that brake system designers must overcome. As moisture increases in the brake fluid, it collects around the parts of the brake system that are susceptible to corrosion, and these parts begin to be at risk.1 Corrosion can be a big deal in brake systems, as anyone who has restored a vintage vehicle will attest. Nasty.

Glycol: Idiot-Proof Brake Fluid
Early on, brake system designers considered these issues, and to reduce the potential issues associated with excess moisture, almost all vehicle manufacturers today use and specify a kind of brake fluid that is designed to tolerate significant amounts of moisture literally by assimilating it, rendering it appreciably less harmful.2 Enter glycol brake fluid. So far so good.

Eventually, however, because of this designed-in water management, the fluid gets overloaded and must be replaced for braking to stay effective. So important is this fluid replacement point that vehicle manufacturers have traditionally (virtually from the beginning) called for a maximum of a two year change interval. By that time, the fluid will have started to turn golden, then light brown, indicating that it has absorbed progressively more moisture. Eventually, if left unchanged beyond the recommended service interval, the fluid will become dark brown, indicating high amounts of water absorption and thus badly contaminated and goopy, slimy fluid. It's at this point that the fluid not only is dark, but is actually dangerous, more grease than fluid. The sad part is, most systems don't have their fluid replaced even then, and this is why manufacturers like glycol fluid. It's still working even then.3 Sadly, the days when you could ignore brake fluid changing with impunity are over. Today's anti-lock braking systems (ABS) is why. The steel valving in many ABS pumps does not tolerate the moisture buildup, resulting in serious system malfunction.4

DoT, What's That?
The importance of monitoring brake fluid's water content is clearly demonstrated by the fact that fluids are rated by the U.S. Department of Transportation (an arm of the National Highway Traffic Safety Administration or NHTSA). The DoT's rating numerically communicates how contaminated with water the fluid can be and still work properly. In practical terms, how able to be neglected it is. The arbitrary numbers 3, 4, and 5 indicate a brake fluid's ability to resist boiling into a very compressible gas even when new (all glycol brake fluids have both a new and used boiling point specification, commonly called "dry" and "wet").5 Thus the DoT number is directly tied to how successfully the fluid tolerates moisture.
Not just the aluminum brake parts are corroded by glycol brake fluid. Note this brake banjo bolt, stripped of its zinc plating exactly where it contacts the brake fluid.

DoT 3 vs. DoT 4 vs. DoT 5.1
DoT 3 brake fluid was the standard for many years but is now obsolete. DoT 4 emerged with the advent of sintered metal brake pads during the early 1980s (first on police bikes, later on everything -- note the heat shims on the backs of your pads), and has replaced it, being still a glycol fluid but with a slightly higher boiling point. Glycol brake fluid containers now are labeled "DoT 3/4," as the two fluids are virtually identical and eminently interchangeable (though manufacturers insist DoT 4 is preferred, and with all pads now at least partly metallic it theoretically is...). However, this dual rating seems to have merely confused everyone. Technically, DoT 3 should be used only with organic brake pads (exclusively aftermarket, and hard to find). DoT 5.1, a glycol fluid, was develped to give ABS systems the heat rating of silicone but with glycol's preferred lubricity, something that wasn't possible until fairly recently. More on that later. DoT 5.1 then is actually the highest rated glycol fluid available today, and that chosen by most folks seeking the best available.

The Good and Bad of Glycol Brake Fluid
Glycol brake fluid is ideally suited to the realities of everyday vehicle ownership. It turns telltale color in direct proportion to moisture content and assimilates that moisture so as to mitigate its effect. Glycol brake fluid is clearly extremely forgiving in this sense. Dumbed down, user-friendly. In fact, vehicle manufacturers use it because they are quite aware that the average owner will literally never -- yes never -- change his brake fluid, let alone do so at the recommended maximum two-year intervals! Additionally, glycol fluid's natural lubricity is good for reducing friction in ABS systems' rapidly moving metal valving. So the most common brake fluid in the world is so because it is pretty much goof-proof. The lowest common denominator, and the best known: i.e. "McBrake Fluid."

There are well-known disadvantages to glycol brake fluid however. First, the very attribute that enables glycol brake fluid to tolerate moisture actually causes it to attract that moisture, as any alcohol product will. For this reason, brake fluid suppliers recommend that only small amounts be kept on hand, that a tight seal be kept on any unused fluid (In the old days, it used to be available only in metal cans), and that leftover fluid be disposed of after only a short time. Another disadvantage, and a significant one, is that glycol fluid is chemically caustic, meaning that it damages other materials. It effortlessly removes paint and does strange things to plastic. Even after it is wiped off, glycol fluid causes catalytic embrittlement, a chemical reaction that though not apparent at first suddenly makes deep cracks appear. The plastic used in motorcycle bodywork is especially vulnerable.

The Good and Bad of Silicone Brake Fluid
Some time in the late 1960s the U.S. Army commissioned the development of a brake fluid that would not promote heavy corrosion in lightly-used vehicles. Dow Corning is said to have played a major role in that, giving the world the alternative to glycol brake fluid, silicone brake fluid. Not only does silicone fluid not promote corrosion, it also will not harm paint or plastic. On the disadvantage side however, silicone fluid aerates very easily, and in fact its only real downsides are all related to this. More on that later. Harley-Davidson, until 2005 probably the best known adherent of silicone fluid, warns owners of their older models to let the fluid sit at least an hour before using it. The trip home in the saddlebag is enough to aerate silicone brake fluid and cause it to be unusable until it has sat for a couple hours. Other issues arise from this aeration tendancy also, such as the inability to properly bleed silicone fluid by the traditional pump-and-bleed method, even when the fluid has not noticeably aerated. Moreover, silicone fluid does not change color as dramatically as does glycol, to tip the user to its moisture content, and theoretically at least, does not at all tolerate, that is, disperse, moisture, making, it is said, systems using it more local-corrosion prone over very long periods. This last point seems to not be very much observed in real life however. Silicone brake fluid also lacks glycol fluid's naturally occuring lubricity. This and its slightly higher viscosity make it incompatible with the mechanical valving in some antilock braking systems. Too bad, considering the corrosion trouble glycol fluid is presenting in ABS today.
This mixture of glycol and silicone brake fluids shows how, much like water and oil, the two do not mix. Moreover, note the layers. The glycol fluid is at the bottom, the silicone on top. Silicone fluid then is both more viscous and lower specific gravity than glycol fluid.

Beware that silicone does not play well with glycol. The two don't mix, looking much like oil and water. Even traces left in the brake hose when switching over is a concern. The changing over should be done by disassembly, not merely flushing through, to avoid any contamination that may account for most of the reported changeover issues. (Interestingly, many today report few if any issues with mere flushing, though I go so far as to disassemble and ultrasonic clean everything, even the hoses.)

During the 1970s when DoT 3 was the highest performing fluid available, racers suddenly discovered silicone fluid and since it had a higher heat rating it was assumed to be a higher perfoming product, something racers are always on the lookout for. It didn't take long however for reports to surface of less consistent braking force and today, no one believes silicone fluid to be up to the rigors of high performance use, however adequate for non-racing. Silicone fluid's aforementioned low resistance to aeration is the reason. Also, car collectors have discovered that silicone fluid when leaking into a car's vacuum operated brake booster can enter the engine and quickly fraction into silicate, i.e. sand, not a very good thing for the engine. For these and other reasons, virtually all aftermarket brake parts suppliers (mostly car oriented) strongly discourage silicone fluid's use.

Today brake fluid choice comes down to glycol vs. silicone. It therefore makes little sense to speak of brake fluid in terms of DoT numbers. The larger issue is fluid composition, not boiling point. Virtually all vehicle manufacturers specify glycol. Here's another reason to think in terms of composition, not numbers. Reportedly, Europe did not have a hand in developing silicone fluid and has consequently never acknowledged it. Thus, Euro motorcycles having a "DoT 5" brake fluid specification may in fact be referring instead to 5.1, the top performing glycol fluid today. Shorthand, in other words. True or no, you can't reliably go by just the numbers. With 5.0 being silicone and 5.1 glycol, this is a very unfortunate bit of confusion indeed. Just think glycol and silicone.

Glycol
Silicone
  • Fairly long official change interval and incredibly long practical one
  • Vehicle inactivity results in heavy aluminum brake cylinder corrosion
  • Is extremely injurious to paint and plastic, even the baked on paint on brake cylinders, and promotes corrosion as well to steel brake parts
  • Self-lubricating quality very good for ABS
  • Extemely consistent pressure under widely varying conditions
  • Deceptively user-friendly as to brake system bleeding
  • Glycol brake fluid cost is very low
  • No hard and fast required change intervals
  • No corrosion effect on aluminum brake parts
  • Does not at all injure paint, plastic, or steel brake parts
  • Lack of self-lubricating quality bad for ABS
  • A reputation in racing conditions of maintaining pressure less consistently than glycol
  • Frustratingly difficult to bleed when setting up or servicing system
  • Silcone brake fluid is considerably more expensive (and will probably get more expensive with its decreasing powersports industry support).

Emotion in the Mix
One thing this subject has been and still is, is charged woth emotion. Think of it as equal to oil threads on user forums. Even on the inside of the powersports industry, where I have been all my life, brake fluid rhetoric takes the place of reason. Virtually all non-Harley dealer mechanics and trade school students are intensely indoctrinated against silicone brake fluid, and everyone has an example to share of a bad scenario. In light of this, here are a few facts that may surprise you.

  • Although silicone fluid localizes moisture instead of assimilating it, tests have shown moisture accumulation in silicone fluid to not be appreciably more than in glycol fluid.
  • Although no one would condone it, brake fluid manufacturers have worked hard to minimize the harmful effects of accidental combination of glycol and silicone fluid, until today, even 50/50 test mixes have not resulted in significantly impaired braking function.
  • Glycol brake fluid attracts and accumulates moisture incredibly quickly. Experts in this field suggest keeping unsealed brake fluid no longer than three months, far less than what has been traditionally recommended.
  • It is now believed that silicone brake fluid acts as a barrier to moisture absorption through rubber brake hoses, a boon to powersports vehicles that are built with more rubber in their brake lines than are cars. This also explains why silicone-filled systems, despite lacking the benefit of glycol's moisture-assimilation, exhibit no more moisture accumulation than do glycol systems.
  • Tests have proven that though silicone fluid is inherently more compressible than glycol fluid (because it aerates easier), most of this added compressibility is at very high temperatures, and thus is not observable in normal street use.
  • That huge bugaboo incompatibility with cylinder seals was actually largely put to rest decades ago. More currently, even seals previously used with one fluid have not been found to react to the other. The only time I personally have seen issues is when the seals were aftermarket, and then the problem was obviously not the fluid, but the very poor quality rubber typical of aftermarket rebuild kits.

Final Thoughts
Since originally writing this article in the late 1980s, it has become one of the most-read brake fluid articles on the 'net and has resulted in a number of contacts from folks surprisingly in the know on the subject, including a former Dow Corning engineer. I used to take a hard line regarding silicone fluid because I was trained by powersports OEMs, who are almost universally adamant about glycol. So I drank the glycol Kool-Aid, so to speak. But I have since changed my view. You can't fully appreciate how much of a change of view this is without understanding how diehard Honda I once was. But I have had to face the facts.

Vintage car owners love silicone brake fluid. It's nearly synonymous with Chevelle and Corvette. They report only the aforementioned difficult bleeding issue, and are thrilled with the lack of caliper corrosion that is so common with glycol fluids. This for many is the bottom line. Also, silicone fluid is reportedly still specified by some government agencies. Harley-Davidson built their products with silicone fluid for many years, in mostly their Kelsy-Hayes brake systems, finally changing to glycol fluid in 2005 (2006 in the Sportster), just a few years (1999) after finally discovering opposed piston brakes. After all the training I have had, all the experience, and very recently much trial and error, there is only one negative about silicone fluid I can find. It is much harder, at times very much, to introduce into the lines and properly bleed, absolutely requiring either a vacuum or pressure tool to make bleeding work. This is widely understood, come to find out. Silicone fluid's propensity to easily aerate, its slightly thicker viscosity, and its lack of the accustomed glycol fluid's self-correcting nature in the presence of air or moisture, make setting up a silicone system for the first time potentially very frustrating, and at any time a lot more work than doing the same thing with glycol. Pressure or vacuum bleeding is the only way. But I have to say I can detect no performance difference and am pleased to know the bike's aluminum brake cylinders are no longer steadily eating themselves away. Life is good. Obviously, there is nothing wrong with glycol brake fluid and both do a great job in today's vehicles. However, here is where I am today. In the vintage powersports world, in which a huge percentage of bikes are typically ridden less than 5,000 miles in a year and brake cylinder corrosion is the prime issue, the advantage falls clearly on the silicone side. I welcome your comments.

Recommended Reading
Brake Squeal and Other Ills
ABS Explained
An intelligent view from Moss Motors
Interesting findings re silicone and brake boosters
The performance side of things
One man's thoughtful, reasoned analysis

Footnotes
1. The steel brake pistons are in view here, though in powersports vehicles their nickel plating comes off way before anything else happens to them.
2. Common designed-for figures seem to be as much as 3 percent, which seems like quite a lot.
3. For over 35 years I trained powersports mechanics. Innumerable times I asked groups for anyone who changed their brake fluid per the two year specification. Not once in all that time did anyone raise their hand. And this is mechanics, presumably folks more maintenance concientious than average,
4. Everyone knows ABS is supposed to default to a standard system when a fault happens. What is not generally considered is this is true, and universally so, but only on regard to an electronic failure. Mechanical failures such as rusted and seized brake caliper pistons don't apply.
5. Because silicone brake fluid does not assimilate water, its DoT specification is only one part, that is, dry. It has no wet rating.

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© 1996-2016 Mike Nixon