DOT 5

Discussion in 'Original Drivetrain Topic' started by l16pilot, Sep 3, 2018.

  1. l16pilot

    l16pilot Veteran Member Lifetime Gold Member

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    The topic of running DOT 5 silicon fluid in new/rebuilt brake systems has been discussed with it seems, split opinions. Personally, I've run DOT 5 in my cars since the mid-90s without any issue whatsoever, but recently spoke to Lone Star Caliper about the M/C and calipers they rebuilt for me and they recently changed their position on the use of DOT 5 due to a high number of customer returns. They did their own investigation and determined the formula for DOT 5 changed somewhere in the 2015 time frame and it now swells the seals. I was able to find some old DOT 5 dated 2013 and older and asked Lone Star to send me a set of seals along with my rebuilt components. What I received was a variety of seals...two of which I could identify as MC seals and the other two as similar chevron seals, but larger in diameter. I did not receive any caliper seals.

    I sectioned the seals and soaked them in two different MFGR's DOT 5 for a week and then compared them to the virgin sections.
    These are the two DOT 5 fluids used. The Fade Free is dated 2004 and the Super S is dated 2013.
    Dot5 1.jpg
    Comparison of the various seals received from Lone Star.
    Dot5 2.jpg
    Interesting that the two un-identified chevron seals DID swell approximately 50% cross section when subjected to both DOT 5 fluids felt noticeably softer and more "spongy". The two seals I believe do apply to my 70Z M/C did not swell at all and there was no noticeable change in rigidity or feel.
    Dot5 3.jpg

    As a final check, I subjected additional sections of all four seals to DOT 3 for a week and there was no noticeable change to any of the seals as a result.

    I was hoping the test would be definitive by testing DOT 5 prior to when Lone Star states the formulation change took place...but it does not. Instead, to me, it only sheds more doubt on the compatibility of DOT 5 in our brake systems. Draw your own conclusions.
     
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  2. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Thanks Steve for the excellent analysis and write-up.

    Seems like by now someone would sell a complete seal kit that is compatible with DOT5. The materials are out there. The seal sizes are standardized enough to make it worthwhile. It may even be possible to use the existing tooling and just substitute compounds. For some shapes a lathe cut seal is even acceptable. Maybe the water absorption fear (or lack of) may be keeping folks from considering DOT5.

    Here is a link to a chemical compatibility guide:

    /https://cdn2.hubspot.net/hub/163881/docs/elastomer_chemical_compatibility_chart.pdf?t=1408391818091

    Go to page 5 for the Brake Fluid listings. I'll assume that petroleum based is DOT3 and synthetic base is silicone DOT5. Silicone has issues with the typical Buna-N/Nitrile-NBR seal materials which is what you may be seeing in your analysis.

    EDIT: Found a better guide that specifically lists the type of brake fluids:

    http://www.ecosealthailand.com/images/catalog/o_ring/Elastomer guide chemical compatibility.pdf

    Go to page 27.

    As you can see, there are fewer seal materials listed as compatible with silicone fluids, but they are out there. Some materials are VERY expensive. I spec'd out a seal made from Parker material one time until I found out the seal cost over 200 dollars for a 3 inch diameter seal!

    Here is another good article on the subject I bookmarked:

    http://www.adlersantiqueautos.com/articles/brake1.html
     
    Last edited: Sep 3, 2018
  3. ol' grouch

    ol' grouch Veteran Member

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    Excellent research. When DOT 5 first came out, I changed a couple of cars over for people. Very labor intensive as I flushed the old lines with alcohol and then blew it out with air. Something the Second Gen's don't have but doesn't like DOT 5 is the pressure brake light switches on older cars. I replaced switches three times on a '58 T-Bird. They all began to seep and lose fluid. I don't see any advantage to DOT 5.
     
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  4. l16pilot

    l16pilot Veteran Member Lifetime Gold Member

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    Dave - thanks for the kudos...what really surprised me were that two of the seals were apparently unaffected by the DOT 5....but I agree, should not be a hard-to-overcome issue for the supplier of the seals.
     
  5. 70 SS/L78

    70 SS/L78 Veteran Member Lifetime Gold Member

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    Excellent work Steve! When I restored my car I contemplated using DOT 5 but ended up DOT 3 at the recommendation of the tech at Brake & Equipment. They rebuilt both my master & calipers. Main advantage of DOT 5 is not having the fluid destroy any of your painted parts.
     
  6. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Sometimes it is the little things that don't you immediately think of that can get you:
    (From the tech article I linked to)

    Vacuum Boosters Take a sample of silicone (I used gasket sealer years ago) and burn it with an acetylene torch. It oxidizes to a fine white powder. This is sand. If you have a vacuum-brake booster, some leakage into the engine is possible, so silicone brake fluid is not for you. Use glycol DOT 3 fluid to avoid an engine disaster. Any potential good from silicone is far outweighed by massive engine problems if it gets past the booster seals.
    Use glycol DOT 3 fluid if you have a vacuum booster.

    I also contemplated using DOT 5 but will most likely go with DOT3.
     
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  7. Zstar

    Zstar Veteran Member

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    Nothing I have read justifies the use of DOT 5. Dot 3/4 for simplicity.
     
  8. ol' grouch

    ol' grouch Veteran Member

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    Another "supposed" advantage to DOT 5 is it's hydrophobic. Just the opposite of DOT 3. Instead of absorbing moisture into itself, it will have bubbles of water condense in the lines. I've heard of but, to be honest, not seen, water bubbles freezing in the brake lines to disable the brakes.
     
  9. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    This is a bad thing for another reason.

    One of the complaints that is repeated for DOT 5 is never keeping a solid pedal. The theory is that since water never really mixes with the fluid but instead stays as bubbles and that the bubbles when in close proximity to the calipers can easily reach the boiling point thus putting air into the system.
     
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  10. l16pilot

    l16pilot Veteran Member Lifetime Gold Member

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    Has anyone tried Mil Spec 5606 hydraulic fluid used in aviation brake systems? It's mineral based which should work well with Buna N seals. I haven't researched enough to understand specific characteristic differences to DOT 3 but there's little difference between aircraft brake systems and automotive, so just wondering.....
     

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