This topic has always been a hot one, DoT 3 brake fluid versus DoT 4, versus DoT 5. Which is best? Isn't DoT 5 the hot set-up? Just how are they different, and what are the pros and cons of each? Let's explore all that, and bring clarity to at least some of the uncertain aspects.
Air? From Where?
Let's begin at the beginning. A brake system 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. One way or another, a 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 the time, and this is unavoidable. There are two inherent difficulties with moisture inclusion that brake system designers must contend with. First, as moisture increases in the brake fluid, it tends to collect around the system's moving parts, particularly the pistons. If this localized moisture accumulation continues unchecked, the pistons and surrounding areas will begin corroding and brake function will be seriously impaired. Corrosion can be a big deal in brake systems, as anyone who has restored a vintage vehicle will attest. Nasty.
The other problem with this continuous exposure to moisture is that moisture makes the brake fluid more compressible. You don't want any compressibility in a brake system. For every millimeter that the lever or pedal moves, you expect a proportionate movement of the brake pads against the disc. If you don't have that, you have an unreliable and dangerous brake system. Unfortunately, brake fluid becomes more compressible as it increases in water content. Water contains hydrogen, and in response to heat cycles the brake fluid soon has air mixed into it as well as moisture.
Consumer-Friendly Brake Fluid
Early on, brake system designers considered these issues thoroughly. To reduce the effect of both corrosion and compressibility, almost all vehicle manufacturers use and specify a kind of brake fluid that is designed to tolerate significant amounts of moisture by dispersing it evenly throughout the system, thus preventing its concentration in any one area. So the localized water accumulation issue is dealt with at least. So far so good.
What Does the DoT Rating Mean?
Eventually, however, because of this designed-in moisture management, the fluid gets overloaded and must be replaced. So important is this fluid replacement point that vehicle manufacturers have traditionally (virtually from the beginning) called for a maximum of a two year period of use of the fluid. 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 fluid. It's at this point that the fluid not only is dark, but is thicker also, and is beginning to be polutted with the byproducts of corrosion of the metal parts.
The importance of monitoring brake fluid's water content is clearly demonstrated by the fact that a brake fluid's most important classification is determined by the U.S. Department of Transportation (DoT), an arm of the National Highway Traffic Safety Administration (NHTSA). The DoT's rating numerically communicates how contaminated with water the fluid can be and still work properly. In essence, how able to be neglected it is. This is where the arbitrary numbers 3, 4, and 5 actually come from. The DoT's rating focuses on the compressibility issue, and measures a brake fluid's ability to resist boiling into a very compressible gas even when new (all fluids have both a new and used boiling point specification, commonly called dry and wet). This of course reflects upon its compressibility over time as well, after it has aborbed moisture and become incrementally more compressible. This is the fluid's so-called boiling point, which for most brake fluids centers between 400-500 degrees Fahrenheit when new, and rapidly decreases with water content.
Incidentally, DoT 3 brake fluid is obsolete. DoT 4, a rating that came about in response to the emergence of sintered metal brake pads during the early 1980s (first on police bikes, later on everything), 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," presumably to eliminate confusion, as the two fluids are virtually identical except for 4's more advanced formulation and slightly higher boiling point. However, this dual rating seems to have merely confused people. The short of it is, if using purely organic (exclusively aftermarket, and hard to find today) brake pads, DoT 3 will theoretically suffice. But as a practical matter the DoT 3 fluid container is liable to be so old it has long since absorbed considerable moisture.
Glycol Brake Fluids
Virtually all vehicle manufacturers specify one kind of brake fluid, whose base is an alcohol similar to that in engine coolant. As we have already explored, this eminently ubiquitous alcohol (actually glycol) brake fluid is suited to the realities of vehicle ownership, because it turns telltale color in direct proportion to moisture content and disperses moisture so that it doesn't localize. Glycol brake fluid is clearly extremely forgiving in this sense. In fact, vehicle manufacturers use it because they are quite aware that the average owner will *never* change his brake fluid, let alone do so at the recommended maximum two-year intervals!
There are disadvantages to glycol brake fluid however. For one thing, the very attribute that enables it 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 on the molecular level that leads quickly to deep cracks. The ABS plastic used in motorcycle bodywork is especially vulnerable.)
Silicone Brake Fluids
In years past, all brake fluids were glycol. Then the U.S. Army commissioned silicone fluid for use in vehicles it was having problems on with heavy corrosion, and Dow Corning is said to have played a major role in that, giving the world DoT 5 fluid. The word is that the army has since reevaluated its use, but silicone fluid continues to have adherents. Silicone brake fluid has properties very different from glycol fluid, and has its own pros and cons. On the advantage side, silicone fluid will not harm paint or plastic (well, plastic, true, but modern silicone fluids seem to in fact react with some paints), and does not aggressively attract additional moisture as glycol fluid does. On the disadvantage side however, silicone fluid aerates easily. Harley-Davidson, one of the few current OEM users of silicone fluid, warns buyers 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 until it looks like a freshly poured soft drink. Silicone fluid is also slightly more compressible than glycol fluid, largely due to its tendancy to aerate, does not change color to tip the user to its moisture content, and worst of all, does not at all tolerate, that is, disperse, moisture, making systems using it more local-corrosion prone over very long periods. Silicone brake fluid also lacks glycol fluid's naturally occuring lubricity, making it incompatible with the mechanical valving in some antilock braking systems (which is why DoT 5.1 was developed).
A third brake fluid category could be included, if we were to consider bicycles. Their hydraulic brake systems use mineral oil, that is, more or less, baby oil. About the same consistency as glycol fluid, mineral oil is still not the best thing around paint, but in most other respects it is fairly non-corrosive. Like silicone fluid however, it does not deal well with moisture. Mineral oil is also specified for some accessory powersports clutch controls.
Which is Best?
As you may have noticed by now, instead of looking at brake fluid as DoT 3/4 versus DoT 5, we should see the issue as glycol versus silicone. This represents the larger division of type, and comparing DoT ratings just isn't significant, especially since DoT 5 fluids are now available in glycol formulation. That's right, glycol technology has improved until glycol fluids can now be made to meet DoT 5 standards. DoT 5.1 for example, is a glycol fluid designed for certain ABS systems having mechanically cycling proportion valves. So now we have DoT 3, 4, 5, and 5.1, with all but the 5 designation being glycol.
The real way to compare brake fluids is by deciding what is important to you. Is silicone fluid's safety around paint and plastic more important than biannual changes and a potentially softer action? Its somewhat higher boiling point is now academic, since DoT 5 glycol (5.1) fluids, which approach it in that respect, are now widely available. You also must ask yourself what kind of storage durations your vehicle will experience, and we'll explore that shortly.
On the more practical side, beware that glycol and silicone brake fluids are hugely incompatible with each other. Mixing even small amounts will create a sludge that looks amazingly like Italian salad dressing and is about as effective as a brake fluid -- meaning, not! Of further consideration is that, in many cases, the seals designed for one experience problems when the other is introduced. This is doumented. The changing over itself must be done by disassembly, not merely flushing through, to avoid any contamination that may account for most of the reported changeover issues.
Glycol Fluid and ABS
The importance of changing brake fluid has always been stressed by both OEMs and repair shops. "Every two years" has been the mantra for as long as I can remember, and it's in virtually every owner's manual out there. With ABS now well represented in most motorcycle manufacturer's lineups, we are beginning to see an interesting -- and scary -- phenonemon. ABS bikes are appearing here and there with totally malfunctioning brake systems, due to old, never-changed brake fluid. The mechanical pumps in ABS systems are especially vulnerable, as their steel valving is unprotected against moisture. One thing to keep in mind also is that while cars have mostly metal in their brake lines and thus don't as readily absorb moisture, powersports venicle brake lines are mostly breathable rubber and thus more rapidly accumulate damaging moisture. Food for thought.
In the 1980s when silicone brake fluid began to be popularized by roadracers, it was not yet known that its pressure was inconsistent in hard use, and today it is interesting to note no racing brake system supplier endorses it. It appears also that the U.S. government has decided it doesn't offer the advantages it once thought, either. Vintage car restorers however continue to use it and report minimal issues, and are ready to accept possible performance disadvantages in return for longer changing intervals than are possible with glycol fluids. That seems to be the bottom line. Long maintenance intervals and less than optimum operation, vs. much shorter intervals and superior operation and feel. Like life, it's all a trade-off. What's important to you. Try as I might, I don't find a clear winner here. I used to take a hard line because I was trained by powersports OEMs, who are almost universally adamant about glycol, and so I drank the glycol Kool-Aid, so to speak. But I have since broadened by view. Many silicone users report great results in infrequently-used vehicles, whereas my vintage Honda's brakes that have sat for five years unused are completely corroded up, white fuzz, stuck hard, the whole deal. One more thing. Although both automotive and powersports manufacturers insist on that two year change interval, the need is worse in powersports vehicles. They sit more, they are in harsher environments, and not the least important, their brake systems have many times more rubber in their lines than do cars', whose brake lines consist largely of steel tubes. Rubber brake hoses allow more air into the system, and give off more contaminants into the system as the rubber ages. Since originally writing this article in the 1990s, it has become one of the most-read brake fluid articles on the 'net, the source for many others' works, and resulted in a number of contacts from folks surprisingly in the know on the subject, including a gentleman claiming to have been on the Dow Corning engineering team that developed the first silicone brake fluid. All of this has caused me to rethink my position. The upshot is I see both sides of the issue better, although not conclusively by any means. And I am going to be giving silicone fluid a try very soon.
Brake Squeal and Other Ills
An intelligent view from Moss Motors
Interesting findings re silicone and brake boosters
The performance side of things