DOT ratings
So, what exactly is the DOT rating telling us? More importantly, what is the DOT rating NOT telling us? A quick look at FMVSS116 ? the US Government?s Specification for brake fluids ? will tell us all we need to know?
As Carroll accurately pointed out, DOT 3 fluids are usually glycol ether based, but that is not because they are required to be. In fact, FMVSS116 makes no mention whatsoever about the chemical compounding of brake fluids ? it simply dictates the fluid physical properties. However, the brake fluid industry has by consensus decreed that glycol ether fluids are the most economical way to meet the requirements, so there you are.
These glycol ether fluids are typically a by-product of the process used to make certain paints and varnishes. By definition, DOT 3 fluids must have a minimum dry boiling point (measured with 0% water by volume) of 401F and a minimum wet boiling point (measured with 3.7% water by volume) of 284F. That?s really about all the specification says as far as the performance enthusiast is concerned.
DOT 4 fluids are also glycol ether based, but have a measure of borate esters thrown in for improved properties including increased dry and wet boiling points. A seldom talked about characteristic though is that because of this chemistry, the DOT 4 fluid will have a more stable and higher boiling point during the early portion of its life, but ironically once the fluid does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT 3. By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 446F and a minimum wet boiling point of 311F.
Does this make DOT 4 fluids better than DOT 3 fluids? Not always. Remember, the boiling points listed are minimums and there are DOT 3 fluids out there with higher boiling points than some DOT 4 fluids. The real differentiating factor should be that if you run a DOT 4 fluid you really should change the fluid more often than if you use a DOT 3, if for no other reason than the rapid fall off in boiling point with time.
We won?t even discuss DOT 5 fluids as they are completely unacceptable to the high-performance enthusiast, but we?ll include them in the following table for completeness.
PROPERTY DOT 3 DOT 4 DOT 5
Dry BP (F)@ 0.0% H2O 401 446 509
Wet BP (F)@ 3.7% H2O 284 311 356
Chemical Composition Glycol Ether Based Glycol Ether /Borate Ester Silicone Based
As a trailing note on the DOT ratings, if your car was designed for a particular type of fluid (especially prior to the development of DOT 4 fluids), you should make every attempt to stick with that fluid! For example, if your car was delivered with DOT 3 fluid, the internal components of the system (seals, brake hoses, and fittings for example) were specifically designed and tested for compatibility with DOT 3. Because DOT 4 fluids contain a different chemical composition, the system may not necessarily react in a positive fashion to the borate esters floating around in the mix.
In other cases, just the difference in viscosity of the two different fluids may cause the seals to wear at different rates. What starts as an annoying squeak might eventually become a torn seal or worse. The examples could go on and on, but the message here is this: it?s fine to upgrade from DOT 3 fluid A to DOT 3 fluid B, but you should think twice (maybe even three times) before switching from DOT 3 fluid A to DOT 4 fluid of any sort.
That said, when dealing with modern hydraulic braking systems a numerically higher DOT rating is typically considered to be compatible with a lower DOT rating (except for DOT 5, of course). Unfortunately, this same generality just isn?t true for most older hydraulic system materials.
