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Thread: 1976 R75/6 brake fluid

  1. #1
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    1976 R75/6 brake fluid

    In my infinite wisdom, to make sure I didnít use any old brake fluid, I threw away all my opened bottles of brake fluid. The reservoir cap states DOT 3 only. It seems to me I was using a higher number but I cannot remember exactly what I was using. What are the acceptable versions and since I am not sure what I am taking out, how thoroughly do I have to clean the system on this change?

  2. #2
    Liaison 20774's Avatar
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    DOT 3 or 4 are totally acceptable...4 having a higher boiling point I believe. What you don't want is DOT 5...I think 5.1 is OK but I wouldn't go there as 4 is wildly available and more than good enough for the braking temps/pressures we see on our bikes.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    DOT 4 is compatible with DOT 3. DOT 4 is better from the standpoint of having a higher boiling point. Both DOT 3 and 4 are hygroscopic, meaning they absorb water. This is usually in the form of water vapor. The effect of the water is to lower the boiling point of the brake fluid.

    So what, you may wonder. The brake fluid is non-compressible. This is good as the fluid is effective in transferring the force the rider exerts on the brake lever (or pedal) to the brake pads. If the brake fluid boils the gas that forms from boiling can be compressed. This (as well as having air in the brake system) results in a mushy feeling brake lever and much less force is transferred to the brake pads. And the bike will definitely not stop as fast as it should. This can be problematic.

    Older Harleys used DOT 5, which is a silicone based brake fluid and is not compatible with the glycol based DOT 3 / 4.

    Further confusion is added by DOT 5.1 which is sort of compatible with DOT 3 /4, but in general it doesn't offer any advantage and I steer clear of this stuff.

    You were wise to dispose of opened brake fluid. Once opened the brake fluid can absorb moisture from the air.

    https://www.autoblog.com/2018/08/06/brake-fluid-101/
    https://en.wikipedia.org/wiki/Brake_fluid

  4. #4
    Nick Kennedy
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    Brake fluid being Hydroscopic is weird.
    Why don't they just use Hydraulic fluid?

  5. #5
    Quote Originally Posted by nickrides View Post
    Brake fluid being Hydroscopic is weird.
    Why don't they just use Hydraulic fluid?
    It often won't take the 300 to 400 plus degree temperatures possible at the wheel calipers without catching on fire. DOT 4 is rated to 446 degrees F for a reason. And of course once combustion starts the gasses (smoke) trapped in the system renders the brake totally and absolutely useless. Never try too hard to outguess the chemists. It rarely works out well.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  6. #6
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    Thanks for the feedback. DOT 3 came up first on the NAPA site ( home delivery) so that is what is going in.

  7. #7
    Chip, DOT 3/4/5.1 all work fine but 5.1 is not worth the extra expense. BTW I am deaccessioning my airhead manual and tools if you want them after this all passes. Chip
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '18 Street Triple RS, 2020 R1250R (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  8. #8
    Nick Kennedy
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    I just hate it when I'm riding and my calipers catch on fire!
    Its hard on the whole bike.
    Thanks Paul for the continuing education.
    I'll continue to use and change out my brake fluid.

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