Bench bleed master cylinder

Discussion in 'The whoa and the sway.' started by Utah455, Jul 16, 2020.

  1. Utah455

    Utah455 Platinum Level Contributor

    I have a new brake booster and master cylinder to install (similar to picture). How do you bench bleed it? Do I need to remove the booster to bleed the master cylinder? Or can I bleed the master cylinder with the booster connected by moving the brake plunger in and out?


  2. JoeBlog

    JoeBlog Platinum Level Contributor

    I’ve usually taken just the reservoir off and held it in a vise as I actuated the plunger. It also made it less cumbersome when installing everything into the car. Others may have a genius idea; since I’m no genius, I go caveman on virtually everything
    Last edited: Jul 17, 2020
    Waterboy, Utah455 and TrunkMonkey like this.
  3. TrunkMonkey

    TrunkMonkey Well-Known Member

    It is very easy to remove the master cylinder from the booster, then install the new booster, bench bleed like Joe said, then carefully install the MC on the booster in the car. (MC being level helps make it easier). But if you have someone to assist, you can do it with everything mounted in the car and prior to connecting the lines. (remember to have all the ports except the those used for the "bench bleed" plugged on the MC)

    I always use a trash bag, cut and laid under booster/MC to protect the inner fender liner and other things from the stoppit juice.
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  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  5. Utah455

    Utah455 Platinum Level Contributor

    Yes, I just bought that same exact kit. Came in yesterday. I just wasn’t sure if I could bench bleed with the booster attached or if I had to separate the booster and master cylinder. I don’t know what to expect if I separate the two.
  6. hugger

    hugger Well-Known Member

    All you gotta do I'd use yer fingers just put it over the ports and let it breath on the in stroke
  7. TrunkMonkey

    TrunkMonkey Well-Known Member

    If you have fluid in the MC, put the cover on and secure the bales, remove the two nuts, break the lines loose at the fittings, and wrap, bag and tape them.

    The MC will slip off, and you might have a spacer plug (drum vs disc MC have a different plunge so the rod into the piston dimple on the back of the MC is either shallow, or deep, and some have the deep piston and a "spacer" that makes it a shallow.


    The "spacer" looks like the one below and would go in the piston on the MC on the left to make it similar to the one on the right. So, if your's has that spacer, it might fall out of the MC, due to the tilt as you remove and install the MC.


    But other than the fluid issue, the removal and installation is only the brake lines, (possible pressure switch wire) and the two nuts securing the MC to the booster, so it's not complex.

    After the pressure bleed, securing the top cover on the cylinder and not pressing the piston during install, you should not loose more than a few drops of fluid, but make sure you have the fender and such covered with plastic. Just in case.
  8. Utah455

    Utah455 Platinum Level Contributor

    @TrunkMonkey Cool Thanks! It’s a disc/drum version and it’s not on the car yet. Brand new in box. Sounds like I can take apart (Easily). Mount booster to car. Then bench bleed with the kit. And mount MC to booster. before all the fluid drains out of it :)
  9. TrunkMonkey

    TrunkMonkey Well-Known Member

    Yes. You won't loose any appreciable amount of fluid.

    When you do the bench bleed, go slow, make sure the tubes stay submerged, as the fluid will go both ways on the press and return stroke. Just keep going slowly until all the bubbles are clear of the tubes.

  10. Ken Warner

    Ken Warner Stand-up Philosopher

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