Just wondering if there is a point to bleeding the brakes if the system is never opened, since it's effectively a closed system. For example, does the chemical change should the fluid reach boiling point release gas? Or should one change the fluid periodically because of some type of degradation, which really falls under the category of flush and not bleed, but the process is similar from what I understand.

2 Answers 2


Brake fluid by nature is hygroscopic, meaning it absorbs water. Even though it is considered a closed system, it doesn't mean it isn't hermetically sealed. Brake fluid will absorb water, have no doubt. You should change your brake fluid once a year or there about.

As the brake fluid absorbs water, it becomes less tolerant of heat, meaning it boils at a lower temperature. When you step on the brakes, you are exchanging motion to create heat, which the brake fluid must be able to tolerate. Here is a chart of the different DOT's used for autos (pulled from this Wikipedia page):

        Dry boiling point   Wet boiling point   Viscosity limit     Primary constituent
DOT 2   190 °C (374 °F)     140 °C (284 °F)      ?                  Castor oil/alcohol
DOT 3   205 °C (401 °F)     140 °C (284 °F)     1500 mm2/s          Glycol Ether
DOT 4   230 °C (446 °F)     155 °C (311 °F)     1800 mm2/s          Glycol Ether/Borate Ester
DOT 5   260 °C (500 °F)     180 °C (356 °F)     900 mm2/s           Silicone
DOT5.1  260 °C (500 °F)     180 °C (356 °F)     900 mm2/s           Glycol Ether/Borate Ester

Brake fluid also needs to be changed to reduce corrosion in the brake lines.

You are right in that the process is pretty much exactly the same between flushing and bleeding. If you change a part which requires you to bleed your system, you may as well flush it while you are at it.

  • This may warrant a separate question, but for passenger vehicles, will the latest DOT suffice, or should I look up the specific DOT for my vehicle?
    – MDMoore313
    Commented Apr 8, 2014 at 18:34
  • 1
    Passenger vehicles usually require either DOT3 or 4. Yes, consult your owners manual, or ask a different question. If you do, ensure you put the year/make/model. Commented Apr 8, 2014 at 18:37
  • Once a year seems a bit much, relate form here indicates a factory call for once every 3 years. Commented Apr 8, 2014 at 19:03
  • 1
    One to two years is the industry standard. If Honda says every three years, so be it. As a further post down that thread says, Change it at the interval that makes you sleep better at night. If you want to wait that long, then do so. If you want to change it every year, there is no issue in it. The brake fluid will continue to absorb water no matter what you do. Commented Apr 8, 2014 at 21:41

I drove my 2001 Bonneville 437,000 miles over the course of 10 years and NEVER changed the brake fluid. Take it for what it's worth.

  • Rite on man. Did you notice a decrease in brake performance over time?
    – MDMoore313
    Commented Apr 20, 2016 at 13:07
  • 3
    That doesn't mean you shouldn't change it. Commented Apr 20, 2016 at 16:30

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