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Question about automotive braking systems

  1. May 19, 2014 #1
    This may not be the proper place to ask this so forgive me if wrong. Can someone direct me to the Federal Reg that "specifically" dictates that hydraulic braking systems MUST have 45 deg. "double" flare connections?

    I'm involved in a protracted forum thread regarding 45 deg double flare on automotive hydraulic braking systems and if it is indeed a DOT requirement.

    Thanks so much
  2. jcsd
  3. May 19, 2014 #2


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    The Department of Transportation (DOT) is covered by Title 49 of the Code of Federal Regulations (CFR), which can be obtained here:


    The regulations are collected by year (since theoretically they are revised yearly)

    The National Highway Traffic Safety Administration (NHTSA) is in charge of promulgating and enforcing what are known as Federal Motor Vehicle Safety Standards (FMVSS) which govern various aspects of the design and construction of all types of automotive vehicles. FMVSS 105 and 106 govern the testing of components used in automotive braking systems, and the text of these FMVSS can be found in Subtitle B, Chapter V, Part 571.

    I've glanced thru both FMVSS 105 and 106 and they set standards for testing various items of brake hardware, but I didn't find an answer to your particular question.

    However, the CFR incorporates by reference a multitude of standards from other bodies like ANSI, ASTM, the SAE, etc. depending on the item or activity being regulated. Perhaps the connections you are looking for are governed by a standard published by the SAE (Society of Automotive Engineers).

    Here are the texts of the two FMVSS:



    Good hunting!
  4. May 19, 2014 #3


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    I don't know anything specifically about the DOT automobile regulations, but I wouldn't expect they would be so prescriptive.

    As an analogy, the regulations for aviation don't prescribe anything about how to design a plane or what you parts or materials you can build it from. They only tell you what things you have to demonstrate to prove it is safe.

    Of course if you want to design a vehicle the easy way, you use parts that have already passed the relevant certification tests. But if nobody did anything except take the easy way out, the best selling car in the USA would still be the Ford Model T.
  5. May 19, 2014 #4
    Thanks for the reply and links. I did drill down into the links and found this:

    "2. Brakes
    Federal Motor Vehicle Safety Standards (FMVSS)
    § 571.106 Brake Hoses
    FMVSS 106 specifies labeling and performance requirements for motor vehicle brake hose, brake hose assemblies, and brake hose end fittings. For reference, this standard only applies to the “flexible” conduit, not rigid tubing. Although not covered in the NHTSA standard, the rigid brake lines should conform to SAE J524 for hydraulic fluid lines, SAE J533b for flares, SAE J512 for fittings, and J1047 for fabrication for rigid hydraulic brake tubing."

    I'm wondering if an AN 37 def flare with tube sleeve would be sufficient to handle the pressure of an automotive braking system.

    Thanks for the reply. Again, any opinion on the viability of an AN 37 deg single flare with tube sleeve for automotive braking systems?
  6. May 19, 2014 #5


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    I have an old 4x4 truck I take out in the woods on occasion so it picks up lots of mud and corrosive gunk. I had my brake lines corrode through in 4 places over the past few years so I put stainless steel lines in and Swagelok compression fittings, good for about 5000 psi. More than enough for the service. One place passed the brakes (2 years ago) but another place failed it this year. Apparently, my brakes don't meet the standard. It's not good enough that they meet ASME code and are made out of stainless steel. I have to have cheap, rusty steel brake lines built to much lower standards (SAE) to pass inspection. <rant>

    So yea, the 37 degree brake lines could easily hold the pressure and would work great (especialy stainless steel). Much better than SAE brake lines. But depending on who you get, the lines could fail inspection.
  7. May 19, 2014 #6


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    JIC are the industrial application of the AN fittings.
    AN = Air force / Navy standard
    SAE J514 is the automotive standard for 37 degree fittings
    SAE J512 is for 45 degree fittings

  8. May 19, 2014 #7


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    I note it says "should," not "shall". Legally, that is only a recommendation, not a instruction.

    From Q_Goest's story, I guess the US approach to vehicle testing is similar to the UK: if you have a "non-standard" vehicle (vintage, home-built, etc), you need to find a specialist tester who actually understands the regulations, not a run-of-the mill box-ticker.
  9. May 20, 2014 #8


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    The 2004 CFR 393.46 Brake tubing and hose connections (f)
    "Splices in tubing if installed on a vehicle after March 7, 1989 must use fittings that meet requirements of SAE Standard J512 - Oct 80 .... "

    That section has been discontinued some years later. For example, the section has no body literature in the 2014 CFR.
  10. May 20, 2014 #9
    I've built several street rods, but this one is ground up. Started with 2 straight 1939 Chevy frame rails and built from there. We (Florida) don't have state inspection so no problem there. I agree, if you build it strong enough to surpass existing regs, that should be sufficient.
  11. May 20, 2014 #10
    Bubble (inverted) flares are OEM on many vehicles these days. Apparently there is no such rule from the DOT.
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