What Does the D.O.T. 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 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.
The importance of monitoring brake fluid's water content is further demonstrated by the fact that in the U.S., a brake fluid's most important classification is determined by the Department of Transportation, an arm of the National Highway Traffic Safety Administration. The D.O.T.'s rating communicates how contaminated with water the fluid can be and still work properly. In essence, how consumer-friendly it is. This is where the numbers 3, 4, and 5 actually come from. The D.O.T.'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. 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 around nearly 500 degrees Fahrenheit when new, and rapidly decreases with water content.
Incidentally, D.O.T. 3 brake fluid is for all practical purposes obsolete. D.O.T. 4, a rating that came about in response to the emergence of sintered metal brake pads during the early 1980s, has replaced it, being still a glycol fluid but with a slightly higher boiling point. Glycol brake fluid containers now are labeled "D.O.T. 3/4," presumably to eliminate confusion, as the two fluids are nearly the same. However, this seems to have merely raised more questions in most people's minds. The short of it is, if using purely organic (almost exclusively aftermarket, and usually, racing) brake pads, D.O.T. 3 will suffice. Otherwise, use D.O.T. 4.
Glycol Brake Fluids
Virtually all vehicle manufacturers specify one kind of brake fluid, whose base is alcohol. As we have already explored, this eminently ubiquitous alcohol (actually glycol) brake fluid is suited to the realities of vehicle ownership. It neutralizes moisture, turns color in direct proportion to moisture content, disperses moisture so that it doesn't concentrate, and resists heat related degradation that leads eventually to increased compressibility. Glycol brake fluid is clearly extremely forgiving, maintenance-wise. 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 accept moisture actually causes it to attract moisture, as any alcohol product will. For this reason, brake fluid suppliers recommend that only small amounts be kept on hand, and that a tight seal be kept on any unused fluid. (In the old days, it used to be available only in metal cans.) 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 susceptible.)
Silicone Brake Fluids
In years past, all brake fluids were glycol. Then D.O.T. 5, a silicone fluid having a higher temperature rating, emerged, initially to meet the higher boiling point requirements of racing use. (Race car brake systems include oil-cooler-like heat exchangers and ceramic pads.) Silicone fluid was able to withstand the most heat of any brake fluid, so it earned a reputation as a racing brake fluid. However, 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, and does not aggressively attract additional moisture as glycol fluid does. On the disadvantage side however, silicone fluid aerates easily. Harley-Davison, one of the sole 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, does not change color to tip the user to its moisture content, and worst of all, neither accepts or disperses moisture, making systems using it more corrosion prone, and requiring much more frequent fluid changes. Silicone brake fluid also lacks glycol fluid's naturally occuring lubricity, making it incompatible with the mechanical valving in some antilock braking systems.
A third brake fluid category could be included, if we were to consider bicycles. Their hydraulic brake systems use mineral oil, that is, 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.
Which is Best?
As you may have noticed by now, instead of looking at brake fluid as D.O.T. 3/4 versus D.O.T. 5, we should see the issue as glycol versus silicone. This represents the larger division of type, and comparing D.O.T. ratings just isn't significant, especially since D.O.T. 5 fluids are now available in glycol formulation. Glycol fluids have improved until they now meet D.O.T. 5 standards. D.O.T. 5.1 for example, is a glycol fluid designed for certain ABS systems having mechanically cycling proportion valves. So now we have D.O.T. 3, 4, 5, and 5.1, with all but the 5 designation being glycol, while the 5 is silicone.
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 yearly changes and a softer action? Its higher boiling point, the reason for its development and at one time its strong suit, is now academic, since D.O.T. 5 glycol (5.1) fluids are now widely available. Glycol fluid therefore is, for most of us, the better brake fluid, and the best just may be the 5.1, if the highest boiling point, which is really a moisture tolerance measurement, matters. On the 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 some cases, the hardware designed for one fluid will not accept the other. Brake caliper and master cylinder seals, hoses, and other parts won't always work correctly when the type of fluid is changed.