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Thread: Brake master cylinder leak

  1. #1

    Brake master cylinder leak

    I thought I read something about using mil-spec o-rings when repairing
    a leaking master cylinder, though I can't seem to find the post now.
    Does anyone have information on replacing the master cylinder o-rings.
    We have had a history of leaks on one particular cylinder. I did
    find the post on rebuilding the master cylinder which I will give to
    my A&P. It's a 1973 Sundowner going in for annual in a week or so.





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  2. #2

    Brake master cylinder leak

    >I thought I read something about using mil-spec o-rings when repairing
    >a leaking master cylinder, though I can't seem to find the post now.

    The correct part numbers are "mil-spec" numbers like MS28775-xxx, where xxx
    is the o-ring size.

    Different mil-spec o-rings are for different materials. Fuel, oil, 5606
    hydraulic fluid, Skydrol hydraulic fluid, etc. may all have a different
    compound and therefore a different mil-spec number.

    Having the correct SIZE of o-ring is secondary to having the correct
    COMPOUND so that it doesn't dissolve, swell up, or react with the fluid
    chemically and create a fire hazard or some other problem.

    Your "bolt and bearing" houses have generic Buna-N o-rings. While these
    may work with red aircraft hydraulic fluid, one has no way of knowing that
    they meet standards for "durometer" (hardness), or even how OLD they are,
    since mil-spec parts have date of production (cure) and shelf life spelled
    out on the package, and bulk o-rings off the distributors shelves are an
    unknown age.

    MS28775 is the correct o-ring compound for 5606 "brake fluid" in our
    Musketeers. Cleveland sells o-ring "OH" kits for each model of master
    cylinder. Gerdes has been gone for a long time, so you end up having to
    measure and match the o-rings based on application.

    Bob Steward, A&P IA
    Birmingham, AL



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