Brake fluid is the unsung hero of our systems. Brake fluid is just about as entertaining and exciting as valve-stems, yet without it we would need to have Hercules-sized legs in order to stop our cars. Brake fluid's primary job is to transmit force from the master cylinder to the calipers, and it does so under some intense pressures and temperatures. This fluid is important enough that the government has regulated and set a minimal standard with 14 properties that are required of every fluid on the market. If the fluid fails to meet even one of the 14 set standards, it will not receive a government DOT rating and cannot be sold as brake fluid.
The Department of Transportation National Highway Safety Administration in Federal Motor Vehicle Safety Standard 116 (FMVSS116) has classified brake fluids into three categories: DOT 3, DOT 4, DOT 5, and now DOT 5.1. If water boils at 212 degrees Fahrenheit and freezes at 32 degrees, brake fluid must function below and above any of the temperatures that will freeze or boil water. In other words, brake fluid must remain viscous and resistant to severe conditions.
It must also absorb water without having an affinity for doing so. Its boiling point and chemical properties must remain stable. That's a lot to ask of a fluid, isn't it? It seems logical that if DOT 3 is good enough, then DOT 4 must be better. And if DOT 4 is better, then DOT 5 and 5.1 must be better still. OK then, why? It all boils down to boiling point. When brake fluid begins to boil (from the heat that's generated in the system during braking), it forms gas bubbles. As these bubbles collect and grow, they form pockets of gas in the brake lines. And since gas compresses much easier than fluid, these gas pockets cause the pedal to compress or become spongy. (This usually occurs during lots of hard braking, such as coming down a mountain or on a road course.) This is what is known as brake fade. Each respective class of fluid must meet a minimal wet and dry boiling point. The term "wet" brake fluid means it's saturated with water, while "dry" brake fluid means it's still pure and has no water absorption yet. By the very nature and chemical makeup of brake fluid, it will absorb water no matter what climate it's in.
The different DOT spec fluids have chemical or formulation differences, as well as boiling point differences. There are (at this point) three commercial formulations for brake fluids; two are glycol-based, and the third is silicone-based. Silicone-based fluids absorb no water and should never be mixed with glycol ether-based fluids. DOT 3 brake fluids are a mixture of polyalkylene glycol ether and other glycols (all start life as ethylene glycol-antifreeze); DOT 4 fluids add borate esters to raise the boiling point. DOT 5 is silicone oil-based with additives. DOT 5.1 fluids have essentially the same chemical makeup as DOT 4, only they have as high a boiling point as DOT 5, which is silicone-based. DOT 3, 4, and 5.1 can be safely mixed and used together because they're made of the same compounds, while a DOT 5 mixture should never be mixed with the others due to its silicone makeup. When was the last time you remember changing the brake fluid in your vehicle? The average enthusiast should change his fluid every 1-2 years just to keep the boiling point up.