DOT Brake Fluids

Discussion in 'Brakes, Tires and Wheels' started by glider., Oct 27, 2007.

  1. glider.

    glider. BOT Machine

    Some facts about brake fluids.

    DOT 5 brake fluid is a silicone-based fluid and is not compatible with DOT 4. It is also less suitable than DOT 4 for ABS systems, because of its higher compressibility.

    DOT 5.1 brake fluid is a low-viscosity, high-temperature fluid compatible with DOT 4 and DOT 3. Its low viscosity makes it ideally suited for ABS systems, where it helps the system modulate the braking pressure more easily. Its higher wet and dry boiling points should make it more resistant to brake fade under heavy use. I have had no problems with the braking system since installing this fluid.

    DOT 5 and DOT 5.1 are very different standards, confusingly named by the US Department Of Transportation. Of the two, only DOT 5.1 is compatible with DOT 4. Do not use DOT 5 fluid unless you have a good reason and know how to purge the system of all DOT 4 fluid before hand (this usually requires complete dismantling of the braking system and cleaning of the individual components with a suitable solvent). Mixing DOT 5 fluid with any traces of DOT 4 will form precipitants that will clog the braking system.

    Harley sells DOT 5 Brake Fluid with the following disclaimer: "Fits all models (except '05-later Touring models, '06-later VRSC™, Dyna®, Softail® and '07 Sportster® models)."

    Harley sells DOT 4 with this disclaimer: "Fits '05-later Touring and '06-later VRSC™, Dyna® and Softail® and '07 Sportster® models."

    Here's an interesting read on the fluids.

    D.O.T. 5 Brake Fluid

    DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 4 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concerns in racecars or motorcycles.
  2. DanX

    DanX New Member

    This helps explain why I have strange objects in my reservoir. I paid the local HD dealer to change my tires and brake pads last year. I didn’t like the job they did since my bike was a caddy when I dropped it off and gradually became a very rough ride. I recently changed my tires and replaced my rear wheel with a new stock laced one and during the process found I need to flush my brake system. I would guess that the service tech used the incorrect fluid when he did my brakes. I wouldn’t have guessed that Harley used anything other than DOT 5.

    One more reason not to let them service my HOG anymore.
  3. glider.

    glider. BOT Machine

    Just because someone works at a harley dealer doesn't mean they know what they are doing.:s

    That system has to be disassembled and cleaned out thoroughly.
  4. Jubal

    Jubal Member

    I will be changing the master cylinder and switch housing on my 2006 fat boy, the master cylinder reads very clearly to use DOT 5 brake fluid but the HD dealer says I could use DOT 4. If this was the case why does HD put on the cover to use DOT 5 ?. I think it would be correct to use what the manufacturer says and not the dealer.
  5. glider.

    glider. BOT Machine


    It's your bike but the dealer doesn't know what they are talking about.

    NEVER mix fluids in a system and if you did replace the entire system including lines and calipers, the next person would probably mess it up then not knowing what was in there.
    Stay with what should be in there as it came from the factory and find a dealer that knows what they are talking about.

    4 and 5 DO NOT MIX!
  6. goodole1

    goodole1 New Member

    I realize this is an old thread but I just had an incident where my none stock front brake reservoir has a sight glass on the side not on the cap. I just rebuilt both of my front calipers on my 05 FLHRS Road King and not thinking about the reservoir I followed the spec sheet that came with the rebuild kits for the calipers. They say to use DOT 5 Silicone brake fluid. Well I did and the fluid ate the seal around the sight glass on the reservoir leaked on my tank and destroyed the clear coat. I just put that tank on about a year and a half ago. I can't tell you how upset I am! I bought the seals from HD. The service manual clearly states DOT4 for my year bike, I called HD and they said DOT 5 was fine. So considering all the conclusion is if you rebuild you calipers and have an aftermarket reservoir you better darn well make sure what it's rated for! So now I have to replace the reservoir with a DOT 5 rating to match the seal ratings. HD screwed up big time. I'm going to the dealer tomorrow and hash this (EDIT) out in the meantime I hope my comprehensive coverage covers the tank. $500 is lot of money plus another 200 for the reservoir. (EDIT) .

    Please read this...

    HDTimeline Language Policy - Harley Davidson Community

    Last edited by a moderator: Nov 25, 2013
  7. scootin

    scootin Member

    Yes, I have heard of that happening before. You have to be careful about sticking to manufacturer spec'd fluids when using them on OEM equipment, such as the reservoir
  8. hogcowboy

    hogcowboy Active Member

    My simple brain says don't mix to begin with. Why would you. The stuff is not expensive. Why would you try to cut corners on a safety product. The reason to mix has never entered my mind. Why would you? I don't understand.(baffled)
  9. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    I read this again and I am a wee bit confused as dot 5 is silicone based fluid and I my experience is safe to seals and paintwork
    However dot 4 is glycol based which is a type of alcohol which is very good at stripping paint and eating some seals
    Dot 4 is required by abs systems as it resists compression better than dot 5

  10. dbmg

    dbmg Guest

    Dot 4 replaces Dot 3. Dot 5 is silicone and should never be introduced into a non Dot 5 rated brake system.
    A good read: