Brake Fluid - What Do We Need to Know?

Brake fluid is the elixir that allows your brakes to work correctly. 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 components, have good lubricity for master-cylinder and caliper pistons seals, offer corrosion resistance, and also have a very high boiling point.

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 miscible, while DOT 5 brake fluid is silicone based and can't be mixed with any other type of fluid. Glycol-based fluids are hydrophilic and will attract and absorb moisture out of the air. DOT 5 brake 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 moisture.

The difference is that glycol-based brake fluids will pull moisture out of the air on their own while DOT 5 brake fluid will not, meaning DOT 5 has a much longer service life.
If you're thinking DOT 5 sounds really appealing right now, think again. Expense, as well as the fluid's compressibility and viscosity, makes DOT 5 brake fluid unsuitable for everyday use. So why does it exist? It was created for the military to use in vehicles that will be parked for years at a time. Harley-Davidson used DOT 5 brake fluid until a decade ago but specifies DOT 4 brake fluid now.

A fluid's classification has little to do with its chemical makeup. Rather, it's based on the fluid's boiling point. The minimums for each grade's 'dry' and 'wet' boiling point, with the former state completely free of moisture and the latter containing 3.7 percent water as is common after a year or so of regular use. DOT 3 fluid has the lowest minimum dry boiling temperature at 205º Celsius (140º Celsius wet), while DOT 5.1 has the highest at 270º Celsius (180º Celsius wet).

As the temperature ratings above suggest, any water content in the brake fluid will reduce its boiling point. Boiling brake fluid will make your brake level feel spongy and braking force will be diminished. This condition is known as brake fade or, more specifically, fluid fade. Replacing your brake fluid regularly will help ensure your brakes always perform their best. Most motorcycle manufacturers advice to replace brake fluid every two years, but best to check that with your owner's manual. Tag: brake fluid DOT-3 DOT-4 DOT-5 DOT-5.1 brakes hydrophilic hydrophobic glycol-based
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