I wondered about DOT-4. I've been reading a lot about brake fluid over the past few days, and am trying to sort through some of the information out there. As they say, it's a bit like trying to drink out of a firehose. Here are some of the things that seem to have been said consistently:
- As the DOT number increases, so does the boiling point. Therefore, all other things being equal, a higher DOT number is better.
- Brake fluid is rated DOT-3, -4, -5, and -5.1
- DOT-3, -4, and -5.1 are glycol-based, and are hygroscopic. That is, they absorb water. As they do so, their boiling point decreases.
- DOT-5 is silicone-based, and does not absorb water.
- The glycol-based and silicone-based fluids are not compatible, and must not be mixed.
- The glycol-based fluids have both a "dry" and a "wet" boiling point. The dry boiling point applies to new fluid, and the wet boiling point applies to fluid containing a certain % of moisture. As the moisture content increases, the boiling point drops. It can drop so low that heat from the brakes can cause the contaminated fluid to boil, rendering the brakes ineffective.
- DOT-4 is more hygroscopic than DOT-3.
- DOT-4's wet performance drops off much more quickly than DOT-3's. That is, at some level of water contamination, DOT-4's boiling point will be lower than DOT-3's.
- DOT-4 and DOT-5.1 contain certain elements that can degrade the rubber seals in brake (and clutch) systems designed for DOT-3 fluid.
- If a system were rated for both DOT-3 and DOT-4, I would not hesitate to use DOT-4 if I were to be diligent in bleeding the system annually or so.
Please jump in and correct any of the above that's incorrect! Given that our vehicle specifies DOT-3 fluid, I would worry about using DOT-4, although the LV formula sounds like a good fit for our winters.