Difference between glycol-based (DOT 3, 4, 5.1) and silicone-based (DOT 5) brake fluid.

Everyone likes to brag about horsepower and going fast, but we all know that stopping is pretty important, too, and brake fluid is the magic elixir that allows it to happen.

Brake fluid is responsible for transmitting force from the brake lever to the back of the brake pads. It needs to be non-compressible to effectively transmit pressure, have low viscosity to be compatible with ABS, have good lubricity for master-cylinder and caliper pistons seals, offer corrosion resistance, and also have a very high boiling point. Thatís a lot of responsibility, and some fluids are more responsible than others.

Brake fluid is available in four grades: DOT 3, 4, 5, and 5.1. DOT 3, 4, and 5.1 fluids are glycol based and can be mixed together, but DOT 5 fluid is silicone based and canít be mixed with any other type of fluid. DOT 5.1 confuses people because of its proximity to DOT 5 silicone fluid, and honestly, parts-counter peoplesí lives would be much easier if the DOT 5.1 was just called DOT 6 or even DOT 4.5.

The main difference between glycol-based fluids and silicone-based DOT 5 are that the glycol stuff is hydrophilic, meaning itíll attract and absorb moisture out of the air, while DOT 5 fluid is hydrophobic. But, due to the repeated heating and cooling cycles and the imperfect sealing of master cylinders and calipers, all fluids will eventually ingest some quantity of water. The difference is that glycol-based fluids will pull moisture out of the air on their own while DOT 5 will not, meaning DOT 5 has a much longer service life. Also, while glycol-based fluids are infamous for causing paint to bubble, DOT 5 fluid is harmless to painted surfaces.

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Revised: December 11, 2016.