Brake fitting thread sizes

Brakes, Suspension, Rims and Tires

  1. jake

    jake Senior Member

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    Ok, I switch to an aluminum master cylinder, does anybody know if the rear brake fittng is 9/16x20 or 9/16x18? I cannot get it to stop leaking at the threads. Thanks in advance.
     
  2. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    9/16-18
     
  3. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    But, it doesn't seal at the threads. The flare on the end of the tube does the sealing. The thread just holds it tight against the fitting.
     
  4. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    If the wrong fitting was applied you may have cross threaded the unit to where the flare does not mate properly. Why did you think an aluminum MC was superior in the first place? Cast MC's are pretty much bullet proof unless they rust internally.

    Dave
     
  5. jake

    jake Senior Member

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    Doc diff finally got back to me and confirmed it is 9/16x20, he said that auto part store fittings are too short to make the seal against the flare in the master. I have to find a fitting that is longer. Not sure where to find that.
    2 different cast masters lids have leaked on me.
     
  6. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    If the cast units are new and leak after several years on a car that does not get driven much, you might want to consider going to DOT 5 Silicone brake fluid. This is best done on a complete system rebuild with cylinders, MC and hoses all done at once. I have had this in several of my cars for 20+ years with no leakage. Caveat, they need to be dry and off the dirt. The Chicom MC's that are being imported by Centric and others are POS. The single pot ones are especially bad. Have had two of them crack at the brake line connection, do not buy those anymore.

    Dave
     
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  7. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Geez, I could have sworn it was 9/16-18. I'll have check for myself when I get home. Sorry for any confusion.
     
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  8. Ross Wooldridge

    Ross Wooldridge Senior Member FCBO Gold Member

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    Leaking lids on any master usually mean the rubber seal under the lid is not sealing well against the reservoir rails of the master, and in my experience it's because:

    1) the master body reservoir rails are not flat,
    2) the master body reservoir rails have nicks in them,
    3) the bail (wire clamp) is not adjusted to hold down the lid tightly enough against the rails,
    4) any or all of the above.

    Due to casting tolerances being poor and due to rough handling, the master cylinder bodies are very often not flat, or have damage, right out of the box, especially rebuilds.

    A local grey beard rebuilder here puts his on a belt sander to true them up. I don't have a belt sander (yet), so I true mine up on the bench (DON'T do this on the the car) - I put wadded up damp paper towels in the reservoirs to catch the filings, and use a flat file to clean them up, then I repeatedly vacuum/wash/bench flush the unit to ensure no errant filings are introduced into the cylinder.

    20 + years, no leaks, no damage.
     
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  9. 71NewYorkMan

    71NewYorkMan Member

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    NAPA dealers typically have a good assortment of brake line fittings.
     
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