Author Topic: What Brake Fluid do I use?  (Read 11999 times)

Offline Red01

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What Brake Fluid do I use?
« on: June 07, 2007, 09:25:50 PM »
Most manufacturers specify DOT 4 these days, though there are a few exceptions. Some Harleys specify DOT 5 and KTM specifies mineral based hydraulic fluid in some models, especially in their hydraulic clutches. Older bikes may specify DOT 3.

DOT 5 brake fluid is silicone based and is generally not compatible with any polyethylene glycol based fluids, such as DOT 3, DOT 4 and DOT 5.1. You may find a few brands that are, but if you decide to use it, be sure it is compatible.

It is safe to mix DOT 3, 4 and 5.1 with each other. Mixing a lower DOT level of these three will degrade the boiling point, so it's best to use the same grade or move upscale.

DOT 3, 4 & 5.1 are hygroscopic (absorbs water), and should be flushed regularly. Most sources recommend every 2-3 years - more often in severe conditions, like racing or prolonged exposure to damp environments where it has the chance to absorb atmospheric or environmental water. This also freshens the corrosion protection additives to keep the brake system healthy.

DOT 5 is non-hygroscopic. It will not absorb atmospheric water. However when water gets in DOT 5, it separates from the fluid instead of staying in suspension like the others. Since water is heavier than DOT 5 brake fluid, water will settle in the low spots and can cause corrosion. Also, when the brakes do get hot, the separated water will boil much sooner than the brake fluid and you will get additional air in the system from the steam. While DOT 5 doesn't absorb water, it does absorb air - and it does it on a molecular level. This makes bleeding more difficult and results in a spongier brake feel, even when bleeding is complete. When it gets hot, the effect is magnified. DOT 5 fluid is typically 2-3 times more compressible than DOT 3 & 4 fluids.

If you have a DOT 3 or 4 system and are looking for a brake fluid that meets a higher boiling point than the stock DOT 3 or 4 fluid, then use DOT 5.1 or one of the "Super" DOT 4's. It is generally not recommended to use DOT 5 in a DOT 3 or 4 system unless you completely purge all old fluid and other contaminants (residue from seal wear included). Since there are DOT 3 & 4 compatible super fluids that beat the minimum requirements of DOT 5, it is much easier to use them instead of converting.

If you have a system that calls for DOT 5, use only DOT 5.

Minimum Boiling Point Ranges for DOT rated brake fluids:
Type: Dry/Wet Boiling Points

      DOT 3: 205°C (401°F)/140°C (284°F)
      DOT 4: 230°C (446°F)/155°C (311°F)
      DOT 5: 260°C (500°F)/180°C (356°F) (includes DOT 5.1)

For more brake fluid reading:
2001 GSF1200S
2010 Concours 14ABS