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DIY Front master cylinder sight window repair

Discussion in 'Maintenance and Servicing' started by Tobbera, Feb 27, 2012.

  1. Have you problems with your front master cylinder sight/inspection window leaking brake fluid? I had. Are you reluctant to spend $250 on a new master cylinder for a such trivial issue? Here is how to repair it for coffee money with the help of all-round epoxy glue.

    Material needed is transparent epoxy glue, or even better, proper epoxy resin, used in for example boat repair and building. This is a awesome thing to have at the shelf for all kinds of general repairs and manufacturing, not only on the bike. I did not have proper epoxy resin at hand so I settled with general fast curing epoxy glue from Bunnings, cost about $10.

    How is it done?
    1. Drain the brake fluid via the caliper at the wheel.
    2. Remove the master cylinder from the bike.
    3. Wipe of all grease and fluid from the window and surounding areas with solvent such as terpentine.
    4. Work the same area with a rough sand paper to prepare for a solid bond with the epoxy.
    5. Wipe and clean again.
    6. Place the cylinder perfectly horizontal with the windows facing up.
    7. Mix a couple of milliliters of epoxy on for example a piece of cardboard.
    8. Pour the epoxy into the cradle where the windows is, practically creating a new window out of epoxy. Pout all the way up to the edge for a sleek flush fit. If you cant pour it, use a wood screw and drop it in there.
    9. Let it cure. Use heat to speed curing if needed.
    10. Remount master cylinder on bike and fill up with fresh fluid.


    What would I have done different if I would do it again?
    • Mix the epoxy more gentle to avoid getting bubbles in the "new window".
    • Take the opportunity to total disassemble the master cylinder and maybe replace rubbers, seals etc.
    • Repaint the hole thing.

    Good luck!

  2. You can use a hair dryer on a gentle heat to heat the bubbles out of the epoxy, works very well.

    You did, of course, check to see if epoxy is compatible with brake fluid before you started, right? :-k
  3. Thank for the tip!

    Well, never seen any chemicals beat epoxy. And it has kept sealed for 3 month... :]
  4. Good stuff.