DOT 4 Hydraulic Fluid

I love DOT 5. Man I’ve had in my scoot for 20 years (I should be ashamed) with not a problem. DOT 4 hydraulic fluid is hydrophilic (picks up moisture) and corrodes paint. I’d like to convert my 2005 FLHR to DOT 5. Know they cannot be mixed. What’s involved? Can I wash/rinse my hydraulic brake system with a cleaner (alcohol or the like) and then make the switch or do I have dissemble everything, clean replace some items, and then switch? Dot 4 is supposed to have a better “feel,” well can’t go by me.


The entire system must be taken apart and all rubber components replaced with DOT 5 compatible pieces. You cannot just flush the system or you would be playing with fire. I recently ran into this problem on a car with a proportioning valve, disc front and drum rear. I always had soft pedal because of the small amount of mixed fluids trapped in the valve. Mixing the two will create an oil and water consistency that will heat up quick, evaporate quickly and hold moisture quick but not stop you quick. Don’t run your fluids that long flush and bleed every two years.   Wrench Safe, FMH
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8 Responses to “DOT 4 Hydraulic Fluid”
  1. Mike

    That is some good advice. DOT5 is silicon based and mixing it with any lower DOT number ( petroleum based) causes a chemical reaction.

  2. Edward Beckett
    Edward Beckett

    I believe the proper term should be hygroscopic, which means it will absorb moisture, not hydrophilic, which means it will mix with water or dissolve in water.

  3. David Barnes
    David Barnes

    H-D shop manuals recommend only inspection of level and condition NOT replacement of DOT5 brake fluid. In my 1997 XLH service manual (PN 99484-97)the service table goes to 50,000 miles. Silicone brake fluid is about as inert and stable as a hydraulic fluid can get. Not used in cars because of expense.

  4. Gregg

    If I remember correctly , a 2005 model Harley WOULD already be DOT5 from the start.

    As far as I am aware all of the range switched to DOT4 for 2006 and onwards.

    Something to do with “ABS” compatibility , even tho’ most models did not have ABS for years to come.

    Your master cylinder covers will be marked , but of course they can be changed over the years back and forth.

  5. Dennis sasko
    Dennis sasko

    IAgree with 2 yr bleeding. You will be surprised the gunk that comes out. The following bleeds come out a little cleaner. The job is a little easier when you install RUssel speed bleeders the first time you bleed you system.

  6. Bob Kirner
    Bob Kirner

    If you look up the oem part number for a 2000 fat boy which used dot 5 you will see the same part number for a 2005 . I have used dot five on my 2005 for years with no problem