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Rear Brake not bleeding/Pressurizing

Discussion in 'The Garage' started by Surfrat28, Aug 27, 2018.

  1. Surfrat28

    Surfrat28 XS650 Enthusiast

    Hey guys hope all is well! I have spent most of the day trying multiple things on my rear brake system on my 1980 XS650SG and have had no luck. I have recently purchased a new rear master cylinder from mikesxs and I am still having the same issue. There are no brake oil leaks and I can't get it to fully bleed even with a speed brake bleeder. Any suggestions?

    Ways I have tried to bleed the rear brake:
    - speed pump
    - regularly
    - replace master cylinder
    Every way I have tried there still hasn't been any leaks or pressure to the master cylinder. Could a piston be stuck or is the problem in the brake caliper. Any help would be appreciated.
  2. highboy_coupe

    highboy_coupe XS650 Addict

    The rear on mine is a pig, it took a while, but the standard version works, eventually:

    Bottle with brake fluid, clear pipe to bleed nipple, other end submerged in brake fluid in the bottle
    Pump pedal down by hand, hold at the bottom, release bleed nipple, watch nervously as nothing happens.
    Close bleed nipple, release pedal.
    Observe as nothing happens, repeat 50 times
    Eventually it will firm up and fluid will come out, top up, repeat till solid.
  3. hermanlifer

    hermanlifer XS650 Enthusiast

    I had a hell of a time bleeding mine. A friend suggested I use a syringe hooked to the rear caliper bleeder to draw the fluid through, and it worked in seconds. Just a big plastic syringe hooked to a clear tube to the bleeder. Keep reservoir full and pull pull pull on that syringe till all the bubbles are gone.
    SAFU likes this.
  4. toomuchstuff

    toomuchstuff XS650 Member

    My method is: Half unbolt the MC and tilt it so the line and slope of the piston is uphill. Unbolt the caliper and hang it so the brake line flows uphill to the bleed screw. Have a couple of cold ones and get a good night's sleep. Bleed the next day. If that doesn't get it, compress the piston in the MC and let sit overnight. Always worked for me.
    Good Luck!
    gggGary likes this.
  5. mrtwowheel

    mrtwowheel Honda Etched On Brain Top Contributor

    I like the mityvac . I've had clutches and brakes that would not bleed the normal way. The mityvac is quick and does the job. I do use much more fluid when using the mityvac. Great instructions in the book for motorcycles too.

  6. 5twins

    5twins XS650 Guru Top Contributor

    Yes, I'm a Mityvac fan as well. I've had one for many, many years. I bought it originally to do some vacuum line testing on one of my 4 wheeled vehicles, and from, of all places, J.C.Whitney. I sprung for the deluxe model featuring an all metal pump in a plastic case with all sorts of adapters and fittings. Also included is a small jar that fits in-line for brake bleeding. I use it mostly for brake bleeding now and it does a wonderful job, especially for re-filling empty systems. Once fluid has been pulled through, I revert back to the normal hand pumping to finish up.


    Also of note are my brake fluid "catch cans" - old plastic peanut butter jars with a hole drilled through the lid for the hose to stick through, and then bungee corded to the leg of my mechanics stool so it won't tip over.
    Last edited: Aug 28, 2018
  7. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Perfect advice!
    I might add; assemble the caliper with the piston pushed in just far enough to seal. Close bleeder, dribble brake fluid in though line port til filled then attach banjo and brake line. Now retract piston with a c-clamp, forces fluid through line and MC to reservoir removing air, done.
  8. mrtwowheel

    mrtwowheel Honda Etched On Brain Top Contributor

    I'm a messy brake bleeder, I like this.

  9. As someone who just figured out how to bleed a front brake, and watched someone fix my rear brake, this is what I would tell the OP
    1-buy a Mitivac, they're cheap.
    2-connect one of the tubes to the bleeder, that tube connects to an empty container, and that container connects to the Mitivac.
    3-make sure there's no air leaking out of the system when you pump the Mitivac otherwise you're gonna be super frustrated.
    4-pump the Mitivac and see if guage goes down or holds steady. I use grease all over the nipple, electrical tape on tubes, to help keep air from escaping.
    -now to begin the bleeding
    5-slowly slowly pump down on your brake a few times and hold it down
    6-pump your Mitivac until it gets to 20 psi
    7-crack open the nipple bleeder a little bit and brake fluid should shoot out into the tube, and eventually into the container.
    8-watch the brake fluid level and top it up if it gets too low. don't let it empty or else air gets in.
    9-close the nipple bleeder after 10 seconds or if the psi drops to 10, let go of the brake.
    -repeat steps 5-9 until pressure builds up. you may go through 2 cups of brake fluid or more, but as long as the fluid is leaving the nipple and you keep topping up the reservoir, you should eventually build up the pressure in the brake.
  10. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    The constant irritation with vacuum bleeding is air that gets in at the bleed nipple threads, you always see bubbles, the trapped air is constantly trying to go UP, opposite the mighty vac suction fluid flow.
  11. 5twins

    5twins XS650 Guru Top Contributor

    Yes, that's why I finish the bleeding using several normal hand pumps of the lever, once the system has been filled with the Mityvac. Re-filling an empty system is what the vacuum pump works best for. Trying to do that by manually pumping the MC can take forever. I always anticipate problems when trying to re-fill an empty system but using the Mityvac, I can't even remember the last time I had one.

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