MC Garage Video: Brake Fluid Grades Explained

DOT 3, 4, 5, and 5.1. What’s the difference, and what’s it mean for your motorcycle’s brake system?

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. That’s 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 can’t 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 peoples’ 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 it’ll 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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If you’re thinking DOT 5 sounds really appealing right now, think again. DOT 5’s price, as well as its compressibility and viscosity, make DOT 5 unsuitable for use in motorcycles. 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 until a decade ago but specifies DOT 4 now that all their bikes have ABS.

Okay, back to the common glycol-based fluids then. The difference between DOT 3, 4, and 5.1 comes down to boiling point, and the higher the boiling point, the more abuse and heat the fluid can take. The grade fluid your bike takes will be printed right on the master-cylinder cover.

The Department of Transportation sets 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 401 degrees dry and 284 degrees wet, while DOT 5.1 has the highest at 518 degrees and 356 degrees 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 lever feel spongy and make it harder to stop your bike. This condition is known as brake fade or, more specifically, fluid fade. Replacing your brake fluid regularly (most manufacturers say every two years) will help keep your brakes working their best, and if you’re a beast on the brakes, you might as well upgrade your fluid the next time you bleed the system. Click the link below for a video that shows you how.