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DOT 3/4/5.1 vs DOT 5 and master/piston seals

Discussion in 'General' started by tiggen, Feb 27, 2022.

  1. tiggen

    tiggen Things are lookin' up.

    Are the master and caliper seals in a brake system different in systems that use glycol based brake fluid as opposed to silicone based fluid?

    Ie, can I put DOT 3/4/5.1 in a system that was "designed for DOT 5?" Btw, I was told that the system was "designed for DOT 5," but I have no way of knowing if this true?
     
  2. Motofun352

    Motofun352 Well-Known Member

    As long as the system is clean you can swap.
     
    tiggen likes this.
  3. Daniel06

    Daniel06 Well-Known Member

    My research on the Harley dot 5 systems says that the dot 5 seals will handle non dot 5 fluid, but non dot 5 seals will not handle dot 5 fluids.
    You need to completely flushed the system and clean out with denatured alcohol.
    That's what my internet research found.
     
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  4. lopitt85

    lopitt85 Well-Known Member

    Same here. I'm tracking you have to flush with a solvent in order to completely remove the prior fluid type, then run your choice.
     
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  5. Daniel06

    Daniel06 Well-Known Member

    Yep. Dot 5 will not mix with dot 5.1, 4 or 3.
    If you combine them, will it work? Yes it will. But the brakes will be sub par. They will stop, but no where near like they should. It'll feel like air in the system kinda.
    They say any residual will be like foreign particles in the fluid.
    My Harley's had for 5 and I just flushed with dot 4 and they work fine, BUT IT'S A HARLEY, not crushing lap records.

    On any race or street sportbike if it currently has dot 5 and I was going to switch, I'd pull the pistons out and do a full tear down of the master and calipers. And flush the lines thoroughly.

    To the op: if the system is bone dry and never filled with any fluid, you can use use whatever fluid in dot 5 systems, just don't mix up fluids later.
    If it has been filled with dot 5, rebuild it and clean thoroughly, or just use dot 5.
    Sorry to repeat, just trying to be clear.
    If it's dot 5.1, 4 or 3 seals, or you're not sure, DO NOT use dot 5. I've read that dot 5 will deteriorate non dot 5 seals. I've never experienced this, but why chance it.

    However, I have been running dot 4 fluid in my 2005 Harley sportster that is designed from the factory for dot 5. It's been 1 year 5 months since I changed it and everything is still working fine.
     
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