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  1. #1
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    Silicone-based DOT5: busting the myth

    So just about everybody will tell you that using a silicone-based DOT5 fluid in a brake designed for glycol-based DOT3/4/5.1 fluids is a real bad thing. It will cause the seals to swell and it will react with the old fluid and precipitate, clogging up the system and leading to brake failure.

    I believed in this too until I came across one article and found it quite interesting: Selecting Brake Fluid

    My set of Hope Tech M4 brakes has always suffered from leaky master cylinder diaphragms. No matter what I tried, including replacing the diaphragms, the reservoir caps were always getting wet eventually. It wasn't affecting the actual brake function, but it was quite annoying, especially because I knew that glycol-based fluids are toxic (or at least irritating) and damage paint. It isn't nice to occasionally touch your brake and get your fingers covered in that kind of stuff.

    All right, thought I, being inspired by the article mentioned above, so why don't I try DOT5 then. Even if it will still leak like the DOT5.1 does, then at least it is not toxic (or at least not to the same degree) and it will not damage paint.
    Besides, DOT5 is hydrophobic, so you don't have to refresh it every now and then. It is unclear (only time will tell) if this will make the brakes maintenance-free, but that is a quite possible outcome.
    I felt adventurous after all, so I ordered a bottle of some silicone DOT5 branded "Master" on ebay.

    Before bleeding the brakes, I repeated the tests described in that article to see how the two fluids mix. Then, I drowned some spare Hope seals in DOT5 and waited 24 hours to see what happens.

    Results were good, same as in the article mentioned above. No reaction between the two, no damage to the seals material.

    So I went ahead and bled the rear brake (I'm not gonna touch the front one until at least mid-summer when it should become clear whether the DOT5 is gonna serve okay for me).

    There wasn't anything unusual in the bleeding procedure. I did not flush the old fluid. I simply turned the caliper upside down so that the bleeding port became its lowest point, and the more heavier DOT5.1 eventually drained out, being replaced with DOT5. Some negligible amount of it may have left there, so probably I'll give it another bleed some day.

    After the bleeding the lever feels okay, there's no sign of increased lever throw or spongy feeling due to the higher compressibility of DOT5. No ride report yet (waiting for a better weather), but otherwise so far so good. The brake is functioning properly and there are no leaks.

    To conclude:

    1. Mixing: DOT5 does not react with DOT5.1, or at least it does not precipitate in any kind of gel whatsoever. When mixed by shaking, the fluids will first mix into one, and then quickly separate and stay each on its own with the heavier one (DOT5.1) in the bottom and DOT5 at the top. The only visible effect of mixing them is DOT5.1 stealing the purple dye from DOT5, but both fluids stay perfectly liquid.

    2. Seals: DOT5 does not destroy the seals, at least not in 24 hours (nor in 48+ hours -- I'm still keeping those seals soaking in DOT5 with no visible changes so far). Most likely it will cause no harm to any synthetic rubber seal that you can find in modern brakes, but I can only speak for my set of Hope seals, of course.

    3. Compressibility: I have not found any clearly discernible difference in the lever feel compared to DOT5.1. I think the difference in compressibility is negligible considering the hose and caliper expansion.

    Caveats:

    1. Chemical compatibility of the two fluids was only tested at room temperature. I'm going to heat them up and see what happens. Need to get a suitable glass container for that first, though.

    2. Long term compatibility is not clear. Time will tell. I am going to run the front brake with DOT5.1 and the rear with DOT5 until I feel that it's okay to run both on DOT5, or the contrary.

    3. YMMV. All of the above was tested with Master DOT5, Delphi DOT5.1 and Hope Tech M4 brakes manufactured around the end of 2010.

    If anyone has real practical observations (be it positive or negative) of mixing the two types of fluids and/or using DOT5 in brakes designed for glycol-based fluids, please share.

    Please do not post comments about what brake manufacturers say in the manuals. We all know that very well already, there's no need to repeat it here. Only practical experience (and of course theory, if there are any chemists here who have knowledge on this matter) is what is interesting.

    What is also interesting is the actual materials used to make seals for various brakes currently on the market. Does anyone have any reliable information on this? It will then be possible to simply look them up in those o-ring chemical compatibility charts and see what will and what will not work with different types of brake fluids.

    I will post further updates if/when I have any.

  2. #2
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    Thanks for posting this. I've seen no reason why a rather inert silicone oil should be a problem with the higher grade rubber usually found in brakes; keep us posted.

    I've been thinking of trying the same with some Avid Elixirs when I need to service them; I like the brakes but would far rather not have glycol on my bike. My guess is that silicone would probably work better for keeping the calipers unstuck which seems to be the biggest problem with Avids.

  3. #3
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    Subscribed as I'm interested in how long exactly it's going to take for those seals to disintegrate.
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  4. #4
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    I don't know.

    There might be some mumba jumba going on at a microscopic level. Micro-tears? I dunno. Stay safe and pepper your angus b4 any rides.

  5. #5
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    I would think the seal deterioration might take some time... I'd leave them in there for a few months and see what they look like then? Other than that, great idea, and it sounds like good execution keep us posted

  6. #6
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    How can I subscribe to a thread without posting?

  7. #7
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    for what it is worth, I sold DOT5 fluid in the 80s and 90s. My usual customer was the classic car guy.The advantages of dot5,will not absorb water,will not damage paint,compatible with all seals used in automotive.Disadvantages,if it is not used 100%(meaning no leftover dot3 or 4 ) it is useless,it will evaporate(even in a sealed system it disappears).Having said all that and having used DOT5 in several vehicles I will not use it.I am that rare person that changes his brake fluid in all my stuff every 2 or so years as preventative maint.

  8. #8
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    Quote Originally Posted by MitchD View Post
    Disadvantages,if it is not used 100%(meaning no leftover dot3 or 4 ) it is useless,it will evaporate(even in a sealed system it disappears).
    Very interesting. That's one more thing to see how it goes in my brake, because I didn't remove 100% of the old fluid and there remain traces of it.

    I wonder what exactly happens in terms of chemistry in this case. It sounds quite hard to believe that presence of DOT3/4/5.1 will cause DOT5 to evaporate when they don't even mix.

  9. #9
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    DOT 5
    DOT 5 brake fluid is also known as "silicone" brake fluid. It was designed for the US Army, who has requested permission to stop using it.

    Advantages:

    It does not eat paint or skin.

    It does not absorb water (BUT!!! See more on this under disadvantages, below.)

    It has a higher boiling point than DOT 3 and DOT 4.

    It is compatible with all rubber formulations.

    It is a good choice for Concours cars which are rarely driven and/or never driven hard.

    Disadvantages:

    It does NOT mix with DOT3 or DOT 4 or DOT 5.1. Most reported problems with DOT 5 are probably due to some degree of mixing with other fluid types. A residue of the former non-compatible fluid is sufficient to cause serious issues. The best way to convert to DOT 5 is to totally rebuild the hydraulic system. Additionally, the same advice applies if switching from Dot 5 to any of the other fluids.

    Since DOT 5 does not absorb water, any moisture in the hydraulic system will "puddle" in one place. This can cause localized corrosion in the hydraulics.

    Careful bleeding is required to get all of the air out of the system. It is hard to you without introducing bubbles and thus results in soft pedal feel. These small bubbles will form large bubbles over time. It may be necessary to do a series of bleeds.

    DOT 5 is less compressible (often creating a slightly softer pedal). It is not recommended for racing application.
    It is hard to pour without introducing bubbles and thus results in soft pedal feel,

    It is not recommended by the brake manufacturer, especially for ABS brakes.

    It is about twice as expensive as DOT 4 fluid.

    So silicone fluid aerates easily. Harley-Davison, one of the sole current OEM users of silicone fluid, warns buyers to let the fluid sit at least an hour before using it. If shaken on the way home, it will aerate enough to look like a freshly poured soft drink. Silicone fluid is also slightly more compressible than glycol fluid, does not change color to tip the user to its moisture content, and worst of all, neither accepts or disperses moisture, making systems using it more corrosion prone, and requiring much more frequent fluid changes. Silicone brake fluid also lacks glycol fluid's naturally occurring lubricity, making it incompatible with the mechanical valving in some anti-lock braking systems.

  10. #10
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    You can use 5.1 in hopes system. Its built for it. I dont as I like the 4 better. Just feels a bit nicer.

  11. #11
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    MitchD, so can you elaborate on your experience in using DOT5 in brakes designed for glycol-based fluids and/or mixing the two?

  12. #12
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    MITCHD: So DOT5 kind of acts like Mineral oil right? Mineral oil doesn't absorb water; if shaken will hold onto the bubbles until it has enough time to rest; not sure of compressibility.

  13. #13
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    Update: no seals damage so far. Brake operates normally. Still no ride report though.

  14. #14
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    Quote Originally Posted by abelfonseca View Post
    How can I subscribe to a thread without posting?
    Immediately above the first post in a thread you'll see "thread Tools" with a drop down menu. Clikc on "Subscribe to this thread".

    Tim

  15. #15
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    Quote Originally Posted by MitchD View Post
    DOT 5 is less compressible (often creating a slightly softer pedal).
    Is that supposed to be more compressible?
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  16. #16
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    Here's a quick update.

    Basically, everything's working just fine. Modulation and power are there. The brake feels just great. No seal damage, no leaks, nothing bad, even though I did not completely remove the remnants of DOT 5.1 from the system. In fact, before I changed the brake fluid, both brake levers had been leaking slightly around the screws fixing the reservoir caps. Now, the rear brake, which was bled with DOT 5, is not leaking, and the front one, still with DOT 5.1, is still leaking a little (no matter what I tried, including replacing the diaphragms).

    The only difference is that it feels as though the rear brake's lever is now a little softer when the pads contact the rotor. I'm not absolutely sure on this, because I don't remember how it felt before changing the brake fluid type. I can compare it with the front brake, but its softness could be explained by the longer length of the hose as well. It will be possible to say something definite after bleeding the front brake with DOT5 when I have inspiration and time for that. I think I'll do that soon, because the rear brake has been trouble free so far, and now I feel quite confident about using DOT5.

    Also, the rear brake's lever free travel until the pads hit the rotor is now significantly shorter than it was before. This is a definite plus for me. It will be interesting to see if that will be the same way with the front brake.

  17. #17
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    Having had no issues with the rear brake so far, I decided to bleed the front brake with DOT5 as well. All went perfectly. The brake performs excellent. I have yet to see whether the fluid stops leaking now (it still had been slightly leaking with DOT5.1 until yesterday), as it did with the rear brake.

    Further testing is to come, but I can't see why there could be any trouble in the future.

    Also, there's no problem with the lever feel. I couldn't really tell the difference between DOT5.1 and DOT5 caused by the higher compressibility of the latter.

  18. #18
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    Thanks for the update.
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  19. #19
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    Quote Originally Posted by shapirus View Post
    Having had no issues with the rear brake so far, I decided to bleed the front brake with DOT5 as well. All went perfectly. The brake performs excellent. I have yet to see whether the fluid stops leaking now (it still had been slightly leaking with DOT5.1 until yesterday), as it did with the rear brake.

    Further testing is to come, but I can't see why there could be any trouble in the future.

    Also, there's no problem with the lever feel. I couldn't really tell the difference between DOT5.1 and DOT5 caused by the higher compressibility of the latter.
    I switched out the fluid in soe avid juicy 5's about 2 1/2 years ago. No problems yet. The feel is different, but otherwise problem free. It does absorb a lot of air if you let it so it can get squishy, but I've never had to bleed them.
    The only thing I did different than you was I flushed the system and let them soak in dot 5 for a while before completing the change over.

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