® Brake Fluid
Revision 3, May 2019

Brake fluid basics
Brake fluid. Such a controversial subject! Let's begin at the beginning. A hydraulic 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.

But that means it also has continuous access to moisture, for air naturally has moisture in it. Therefore, brake systems have moisture in them. It's unavoidable. As moisture increases in the brake fluid, it collects around the parts of the system that are susceptible to corrosion. Corrosion can be a big deal in brake systems, as anyone who has restored a vintage vehicle will attest. Nasty.

To reduce the potential issues associated with moisture, vehicle manufacturers specify a type of brake fluid that is designed to tolerate moisture by assimilating it. Eventually, however, the fluid gets overloaded and must be replaced for braking to stay effective. Manufacturers have traditionally called for a maximum of a two year change interval, by which time the fluid will have started to turn golden, then light brown, indicating that it has absorbed progressively more moisture. If left unchanged beyond the recommended service interval, the fluid becomes dark brown, indicating high amounts of water absorption. Not only is the fluid dark, but it is actually dangerous, more grease than fluid. Most of us ignore the fluid even then, but the days when you could do this with impunity are nearly 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 these fluids are rated by the U.S. Department of Transportation (an arm of the National Highway Traffic Safety Administration. 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. Thus the DoT number is directly tied to how successfully the fluid tolerates moisture. In short, DoT numbers are all about moisture. They communicate the usability of the fluid after it has assimilated water, by rating its heat resistance before and after water absorption; why most fluids have two numbers, "wet" and "dry". The fluid's ability to resist heat lessens as more water is absorbed; it can overheat easier and begin to be compressible, which means the brakes can fail. This is more a problem in cars than in bikes, but the hydraulic clutch fluid in some bikes, being attached to the engine, have been observed to fail from heat also.

DoT 3 brake fluid was the standard for many years but is now obsolete. Higher boiling point 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). DoT 5 appeared when silicone fluid was developed, to designate a uniquely high heat brake fluid, then when ABS hit big DoT 5.1 emerged, having the heat range of silicone but being glycol based for the sake of the ABS mechanical valving. DoT 5.1 then is actually the highest temp-rated glycol fluid available today, and that chosen by most folks seeking the best glycol fluid available.

There are faults in the DoT classification system however. First, some countries recognize only three designations, not four as in the U.S.. This makes comparison troublesome. Second, the "boiling" temps are not consistent from fluid manufacturer to manufacturer; they can vary quite a bit. Third, though it's vitally important to monitor your fluid's water content, the DoT system merely hints at this need without telling you what to do about it. That would at least lend practicality. Fourth, the DoT system has mislead consumers into thinking that the classification number is a kind of descriptive labeling useful in comparing and purchasing fluids, which of course it is not. Fifth, the DoT system is basically irrelevant in the case of silicone fluid which neither attracts nor assimilates water. Sixth, brake fluid classifications cross fluid types. That is, both glycol and silicone fluids have been assigned the same number. And there's more. The bottom line is it makes little sense to speak of brake fluid by the DoT numbers 3, 4, 5 and 5.1. It's far more helpful and logical to focus on the fluid's composition: glycol and silicone. The numbers communicate little that is useful; the fluid types tell us everything.

Glycol vs. silicone
Omitting the mineral oil used in bicycles and whatnot, there are just two major brake fluid types: glycol, whose base is related to engine coolant; and silicone, whose composition is akin to the major ingredient in many car "waxes". Glycol is the older fluid, silicone the relative newcomer. Glycol fluid is hygroscopic, meaning it attracts moisture right out of the air, and just as importantly, after attracting it, absorbs it, dispersing it evenly throughout the fluid. It gets this moisture of course through the breather vent in the master cylinder, and also through the brake system's rubber hoses, through which air enters, and thus moisture. For seeming millennia vehicle manufacturers have labored to convince us hygroscopicity is a good thing. They say, better that the moisture is evenly dispersed throughout the brake system than it collect in pockets, because that way the moisture never evaporates to leave compressible air in the system. But this has proven to be a doubtful advantage. At least in pockets it would have opportunity to be cooked out by brake use. Absorbed it can't, and so it continues to accumulate, accelerated by glycol fluid's natural moisture-magnet property, until it ruins the fluid, eventually turning it into mud, and worse, starts to corrode the system's metal parts. Glycol fluid is also fairly caustic, meaning it strips paint and chemically reacts with certain types of plastic, deforming and cracking them, especially that in motorcycle bodywork. Glycol fluid is fairly easy to clean up when spilled. Glycol fluid has high lubricity, encouraging smooth working of brake pistons. Glycol has a natural tendancy toward self-purging of air, making it easy even for the do-it-yourselfer to bleed. Glycol fluid is cheap and readily available. And glycol fluid offers a great visual for when it is absorbing moisture: its color gradually darkens.

Silicone brake fluid on the other hand has very different properties. Commissioned originally by the U.S. Army as part of a maintenance-reducing program and said to have been developed primarily by Dow Corning, it neither attracts nor assimilates moisture. The little water this moisture-neutral fluid does accumulate, less than a tenth of glycol's, by the way, is isolated from the fluid. It does not mix with it. It localizes it. Silicone brake fluid is not caustic. Spilled silicone fluid is more difficult to clean up; soapy water works better than solvent. Silicone fluid aerates easily and thus requires special technique for bleeding. Silicone fluid doesn't have or need glycol fluid's visual mechanism for tracking moisture pollution; i.e. it doesn't darken. Silicone brake fluid is more expensive and not as plentiful. And absent from silicone brake fluid is any natural lubricity, making it unsuitable for the mechanical valving in antilock braking systems, and thus destined to become even harder to find in this ABS age.

Silicone's controversy
Silicone brake fluid is controversial still, even after all these years. The debate revolves around two oft-repeated claims: that silicone brake fluid deteriorates brake system rubber seals, and that it results in reduced brake lever feel.

First, brake system rubber parts. Rubber brake parts incompatibility may be the longest surviving silicone fluid boogieman. However, when using high quality OEM rebuild kits, incompatibility is simply a myth. As is typical with the aftermarket, some aftermarket seals work with silicone, some don't. And even some parts within a given kit work while others don't, which is true also with glycol fluid. This is the way of aftermarket rebuild parts. Avoid them.

Now brake system feel, the other claim. The misconception persists that silicone fluid is noticeably more compressible. However, in my experience such complaints are invariably due to improper installation and bleeding. There is zero difference in brake function or feel between glycol and silicone brake fluids properly installed and in street use. But doesn't silicone brake fluid aerate easily? Yes. It is instructive to look on a bottle of Harley-Davidson silicone brake fluid, where you will find the warning to "allow the bottle to rest on the workbench for an hour before using." Yes, silicone fluid aerates quite easily. Thus bleeding silicone can be a challenge. You can't manhandle it. And in my and many others' experience, the traditional lever-pump-then-valve-open method of bleeding is not effective. Those with experience with silicone fluid use either pressure or vacuum bleeders.

Conclusion
Both fluids are good and have their place. But for vintage brakes that see more storage than use, silicone brake fluid is a boon, despite the Internet's uninformed rhetoric. Silicone eliminates the need to regularly rebuild your infrequently-ridden vintage bike brake cylinders. This is why the stuff was invented, and it continues to be silicone fluid's reason for being. Silicone brake fluid is one of the best things to happen to vintage motorbikes.


Glycol
Silicone
  • Two-year change interval
  • Vehicle inactivity results in heavy aluminum brake cylinder corrosion
  • Extremely injurious to paint and plastic, promotes corrosion as well to steel brake parts
  • Self-lubricating quality very good for ABS
  • Very easy to bleed
  • Low cost and easy availability
  • 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
  • Special technique required to to bleed
  • Considerably more expensive

Notes

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.

6. 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.

7. 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.

8. Harley-Davidson, after decades of being almost exclusive among manufacturers in specifying silicone fluid, has recently moved away from it in response to pressure applied by its vendors.


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.

Purple silicone brake fluid. The color warns and reminds the user: do not put glycol fluid.

This mixture of glycol and silicone brake fluids shows how, much like water and oil, the two do not mix. 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.

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