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New Front brake master cylinder - Dot 5 / Dot 4?

  • galbo950
    galbo950
    12 years ago

     Just received a new chrome front brake master cylinder (allegedly original harley chrome part) in the mail and it states on the filler cap to use Dot 5 fluid, my current original Harley one states use Dot 4 fluid and that's what I've been using. So the question is, do I use Dot 5 or Dot 4? Will Dot 4 affect the new one or if I use Dot 5 as recommended on the cap do I need to do anything to the brake line and caliper other then make sure I flush it out with Dot 5? I know the 2 don't mix. The bike is an 07 Wideglide.

  • 06 Softail
    06 Softail
    12 years ago

     I used dot 4 recently when I fitted 3 inch extension kit to my forward controls. Stated on the mastercylinder cap was use dot 4 only. I did however bleed out all the old brake fluid.  I even asked my local bike shop which one to use, and they said check what it says on the master cylinder. I'm no mechanic, so I can't give a definite answer, but so far so good with my bike.

  • speedzter
    speedzter
    12 years ago

    You should be safe running Dot 4 in the new MC.
    The early (Dot5) and late (Dot 4) MC use the same seal kit, so will handle either fluid.


    I think it would be to hard to completely flush out the old Dot4 if you wanted to use Dot 5, and
    You would also really need to replace the caliper rubber seals.

  • FLHuTChU
    FLHuTChU
    12 years ago

    Use dot 4 only. They went to dot 5 on a few models for a short time and then went back to dot 4. The book recommends to change it every 2 years but depending on how much you ride thats overkill. but its cheap to do. Hope this helps.

    DOT 4:This brake fluid is also based upon poly glycol. Its dry boiling point (ERBP) is 230°C minimum and wet boiling point (Wet ERBP) is 155°C minimum. It is used in medium and higher size of modern cars; and also for vehicles in high-altitude, towing, high-speed braking situations or ABS systems. This brake fluid does not require cleaning the system and it can be mixed with DOT 3,DOT 5.1 without damage to the system. This brake fluid also absorbs moisture out of the atmosphere thus reduces its boiling point (ERBP). It can also damage the paint on a vehicle. The modified poly glycol boric ester DOT4 brake fluid is also available.

     

    DOT 5: This brake fluid is based upon silicone. Its dry boiling point (ERBP) is 260°C minimum and wet boiling point is (Wet ERBP) 180°C. It is used in applications, like for weekend, antique, collector cars that sit for long periods and for some military vehicles. Some car manufacturers are also used in their road driven vehicles. This brake fluid does not mix with DOT 3, DOT 4 or DOT 5.1. It will not absorb water and will not damage the paint on a vehicle. It is also compatible with most rubber formulations. It absorbs more air and giving poor pedal feeling. It is unsuitable for racing due to more compressibility under high temperatures. If as little as one drop of water enters the fluid, severe localized corrosion, freezing or gassing may occur. This can happen because water is heavier and not mixable with silicone fluids thus it is unsuitable for ABS. 

    DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel. Poly glycol type fluids are 2 times less compressible than silicone type fluids, even when heated.

  • galbo950
    galbo950
    12 years ago

     Thanks, I wanted to continue running dot 4 as I still have half a bottle of it on the shelf and i also didn't really want to have to flush the system out. Just wanted to make sure that the Dot 4 would have an adverse effect on a dot 5 master cylinder. I got it off eBay for $134 and it was advertised as a dealer take off and refurbished with a 1 year warranty. Looks brand new and never used, not a mark on it.

  • tussuck
    tussuck
    12 years ago
    Mind you I love my DOT 5.... Has lasted 20 years.
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