The DOT specifications are based on the concept of wet and dry boiling points. The dry boiling point is applicable when fluid is fresh and the wet boiling point after the fluid has been exposed to moisture and has had the opportunity to adsorb water. The minimum values for the wet and dry boiling points are specified for each DOT level, and increase from 3 through 5. Note that these are minimum values, and there is no constraint on by how much a manufacturer may exceed them. To achieve a DOT rating, the fluid must meet both dry and wet boiling point specifications. In addition to DOT level, another important consideration in selecting a brake fluid is the presence of anti-corrosion additives, commonly found in street fluids but not in track fluids.

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.

VPROPERTY |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

While the performance enthusiast is most concerned about boiling points, the government holds no less than fourteen properties of brake fluid in the highest regard. Fail just one of the tests, and the product cannot be legally offered for sale in the US. In order of listing, the properties under the spotlight include:

1. Dry boiling point
2. Wet boiling point
3. Kinematic viscosity (how thick the fluid is, with lower generally considered better for flow)
4. pH value (measure of acidity, with higher generally considered better for corrosion resistance)
5. Chemical stability
6. Corrosion
7. Fluidity and appearance at low temperature
8. Evaporation
9. Water tolerance
10. Compatibility
11. Resistance to oxidation
12. Effects on rubber
13. Stroking properties (lubrication capability)
14. Fluid color

In order to detail each requirement, we would need to reproduce the entire FMVSS116 document here in its entirety. Since that would probably put 99% of all readers to sleep (the regulation is comprised of no less than twenty two pages of brake fluid minutiae), we’ll just provide this handy link:

http://www.nhtsa.dot.gov