Brake fluid has a specific boiling point designed to withstand high temperatures generated during braking. This is the reason why you often hear that race or track car applications tend to replace their fluid with one which withstands higher boiling temperatures. Heat generated is the main contributing factor to brake fade - think of it like the brakes being not as effective as they were initially and not biting as hard on the discs. In addition to fluid, brake pads are usually swapped out for higher temperature items along with slotted or drilled rotors designed to dissipate heat. However, these items will be left for another discussion.
For best results, we recommend changing the brake fluid on road use cars every 12 months and club racing cars every 6 months. Extreme competition cars should change their fluid after each race event as brake fluid can absorb moisture, air or heat up to extreme temperatures and can ultimately lead to brake failure. Something you definitely don't want in a race car.
There are 4 main types of brake fluids which are known as DOT 3, DOT 4, DOT 5 and DOT 5.1. The Department of Transport (DOT) in the United States generated these ratings by using a number of test exercises. The main measurement to note is the boiling point characteristics and these are measured within two ranges, being its dry boiling point and its wet boiling point. Dry boiling point is the point at which the fluid boils when first used out of the bottle and wet boiling Point is a measurement based on 3.7% water absorption in the brake fluid and at what point it starts to boil.
DOT 3 - Usually glycol ether based with a minimum dry boiling point of 205 degrees Celsius and a minimum wet boiling point of 140 degrees Celsius.
DOT 4 - Also glycol ether based with a touch of borate esters to increase its immunity to water absorption. Minimum of 230 degrees Celsius dry and 197 degrees Celsius wet.
DOT 5 - Silicone based with a minimum boiling point of 265 degrees Celsius dry and 180 degrees wet. Being silicone based this type of fluid flows more easily through the pressurized braking system providing greater braking performance and thereby reducing heat. The disadvantage is that by its nature being more compressible it allows more room for air to be present within the fluid.
DOT 5.1 - Glycol ether based fluid has the same dry and wet minimum boiling points as DOT 5. However, being glycol ether based means it doesn’t share the negative feature of silicone based fluids (air storage). These are also known referred to as DOT 4 +. This is therefore the best type of fluid, however, is also the most expensive.