Brakes Part 6: Brake Fluid
So the final step in the braking system is the selection of the brake fluid, but which one DOT3, DOT4, DOT5, DOT5.1 or a manufactures specific type (eg Motul RBF660)? Each type of brake fluid has different properties, making some of them less / more suitable for racing use. A summary of these properties is detailed below:
| Dry Boiling Point | Wet boiling point | Viscosity limit | Primary constituent | |
| DOT 3 | 205 °C (401 °F) | 140 °C (284 °F) | 1,500 mm2/s | Glycol Ether |
| DOT 4 | 230 °C (446 °F) | 155 °C (311 °F) | 1,800 mm2/s | Glycol Ether |
| DOT 5 | 260 °C (500 °F) | 180 °C (356 °F) | 900 mm2/s | Silicone |
| DOT 5.1 | 260 °C (500 °F) | 180 °C (356 °F) | 900 mm2/s | Glycol Ether |
| Motul RBF660 | 325 °C (617 °F) | 204 °C (400 °F) | 1,698 mm2/s | Glycol Ether |
Dry Boiling Point
Tested with 0% water volume.
Wet Boiling Point
Tested with 3.7% water volume.
Viscosity Limit
Tested at -40°F.
Additional Points:
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
DOT 5 fluid specification was expected to be fulfilled by silicone based (SSBF) composition. It was designed for use in applications where its resistance to water absorption (and therefore low corrosion) was desired – like in military equipment. These characteristics were only achieved by unacceptably high compressibility. As such, the DOT 5 grade SSBF is of little value to any conventional automotive or high performance application.
Conclusion:
Taking the above points into consideration, the runs Motul RBF 660 which has the highest dry and wet boiling points and equally importantly is available locally at either DAS Auto Centre or MotorsportWheels.
