How to Bleed a Master Cylinder

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Bleeding a master cylinder is a very straightforward job, but is critical to make sure your brake system is safe and free of air. The problem with air is that it is compressible, while the fluid that goes in your braking system is not. You should bench bleed your master cylinder first, then bleed on the car after installation.

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Method 1
Method 1 of 2:

Bleeding a New Master Cylinder on a Bench

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  1. Bleeding from the bench is much easier than bleeding using the pump method, which takes a very long time and may not work. It is also much less expensive than taking your master cylinder into a mechanic, who will quickly (and expensively) do it with vacuum pumps. It is also necessary if you are installing a new master cylinder. To bleed your master cylinder from the bench, gather the following supplies: [1]
    • Your master cylinder with bleed kit.
    • Fresh brake fluid.
    • A workbench or table with a fixed vise. If you have neither of these, it will be more cost-effective to go to the next method, which does not require a specialized workspace.
    • A wooden or plastic dowel. Make sure this is strong, because you don't want it to snap while you're working.
  2. Set aside the bleed kit that comes with the cylinder, as you will need it later.
    • If the new part didn't come with a reservoir, be sure to remove the one from the old part.
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  3. Steadiness is key when bleeding your master cylinder in this way. Before doing anything else, mount your master cylinder in the vise on your workbench or table, and make sure it is level. [2]
    • Hold the master cylinder by its flared mount, and make sure it is level. This way, the air will leave properly, and it will fill with brake fluid evenly without gaps.
    • It needs to be firmly secured, but not so tight you crush or damage cast aluminum parts. Make sure the plastic fittings are not crushed or blocked when putting the cylinder in the vise.
    • If your table does not have a pre-installed vise clamp, you can buy a separate vise that clamps on to the table if you do not have one pre-installed.
    • If you like the appearance of this table, it might be best not to use it, as vise clamps leave a mark on wood or metal. If you have no other usable tables, wedge a rag in between the vise clamps to avoid leaving a mark. This may not work, so proceed at your own risk.
  4. This should come with your master cylinder, and consists of two rubber hoses and two plastic threaded inserts.
    • The inserts will be threaded on one side, and the other will be a smooth adapter for taking the hose.
    • Check the color of your hose, too. If you have opaque hose, you might want to replace it with clear hose, because then you can see the air bubbles passing through the fluid more easily.
    • You can also opt not to use the bleeding kit, as it isn't necessary. However, this guide will assume that you are using the bleeding kit.
  5. These are located on the side of the cylinder past the flared mount.
    • Install these into the bosses (the threaded holes) without cross threading them. Make sure they are finger tight.
  6. Once you have attached the inserts into the cylinder, you can attach the rubber hoses to these inserts.
  7. This container will be used for catching the excess brake fluid, so make sure you don't mind that it gets dirty.
    • Consider attaching the hoses to the container in some way. Once you begin pumping, they will begin flailing about and spray blake fluid everywhere if they are not anchored down in some way.
    • An old coffee can works especially well for this, as does any cylindrical container with a wide, flared opening.
  8. If your reservoir runs out of brake fluid at any time during your bleed, you'll have to start from the beginning. [3]
    • You should only put clean brake fluid that is less than two years old into the reservoir.
    • Make sure it's in between the "max" and "min" marks and covers the ends of the hoses. Brake fluid is very hygroscopic, which means it absorbs moisture and will degrade, and this damages the seals. Do not reuse brake fluid
  9. Make sure you do this slowly to avoid allowing air into the apparatus, or allowing the wooden dowel to snap.
    • Don't bottom out the piston, otherwise this might allow the entrance of air.
    • You have to pinch the rubber hoses closed each and every time you release pressure on the cylinder.
  10. This will compress the brake fluid, so it will shoot out when you release the hoses.
    • This also prevents air from entering the master cylinder by not providing a vacuum for it to fill.
    • You will know you're finished pumping when there are no more air bubbles rising in the reservoir or present in the container of spent brake fluid.
  14. [4]
    • When installing it in the vehicle, keep it level and remove the bleeding kit as you install the lines. If you did this right, you shouldn't need to bleed the entire system, but you may want to to flush the system of old fluid. [5]
  15. These should have come with the cylinder, and will prevent leakage.
  16. Otherwise brake fluid will leak out from the reservoir.
  17. You need to make sure that your brakes actually work before you begin driving your car.
    • If you've done it right, the brakes should feel clean and crisp when you pump them.
    • If you've done it wrong, the brakes will feel "squishy" when you pump them from your car, indicating there is still air in the master cylinder. If the break pedal feels soft after installing the master cylinder, follow the instructions for the method on bleeding the master cylinder from the car, or this guide on bleeding brake lines .
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Method 2
Method 2 of 2:

Bleeding from the Car

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  1. Unlike with bleeding a separate or new master cylinder, you can do this without gathering new parts or equipment. You will need:
    • A brake bleed screwdriver or wrench. You will have to loosen the screws that hold the hoses in place in order to bleed the master cylinder.
    • Pliers. You will need these to pinch the hose attached to your master cylinder in between pumps
    • WD-40 or other water-displacing solvent. Your brake bleed screw may be caked with oil or other contaminants and be difficult to unscrew. You'll use the WD-40 to remove this material and allow yourself to loosen the screw.
    • A helper. He or she will pump your brakes while you fiddle with the hoses and screws underneath your car.
  2. [6] This will allow you to get underneath it to work with the master cylinder.
    • Immobilize the car by blocking the wheels, and make sure it won't roll by parking it on a flat surface. [7]
  3. This container will be used for catching the excess brake fluid, so make sure you don't mind that it gets dirty.
    • Consider attaching the hoses to the container in some way. Once you begin pumping, they will begin flailing about and spray blake fluid everywhere if they are not anchored down in some way.
    • An old coffee can works especially well for this, as does any cylindrical container with a wide, flared opening.
  4. Have him or her indicate to you what he or she is doing by calling out "down" when pressing the brake pedal and "up" when letting it go.
  5. Now you will begin doing some work on the master cylinder.
  6. This isolates your master cylinder and keeps you from bleeding your brakes as well.
    • You're trying to bleed your master cylinder, and not your brakes, so you don't want to affect the latter by messing with the former.
    • Likely, brake fluid will instantly fly out. This is why you want to have a container attached.
    • Before your helper releases the pedal, make sure you have the hoses back on.
  7. If there is air in the fluid, you will see air bubbles in what was discharged.
    • This is also why it is important to have a container or jar, because without collecting the fluid, you will have no idea whether there is fluid in the brakes or not.
  8. Failing to do so will allow air back in to the cylinder.
    • Don't forget to continue topping up the master cylinder with brake fluid. Otherwise, you may introduce air into it and have to begin again.
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Expert Q&A

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  • Question
    What is bench-bleeding, and how do I do it?
    Evan Ernst
    Classic Automotive Enthusiast
    Evan Ernst is a Classic Automotive Enthusiast based in Spokane, Washington. Evan and his twin brother Nick are the founders of ClassicNation, an automotive enthusiast website. With over 36,000 YouTube subscribers, Evan and Nick share videos covering one of their many classic car restoration projects, an interesting 'featured ride', or helpful how-to repair videos.
    Evan Ernst
    Classic Automotive Enthusiast
    Expert Answer
    Bench-bleeding a master cylinder involves bleeding it outside the car rather than in the car. The process is straightforward. First, fill both sides of the master cylinder—whether it's dual or single chamber—with brake fluid. Next, pump the brakes to move fluid and air through the open valves at each wheel cylinder. Continue pumping the brakes until only fluid, and no air, is coming through. Finally, tighten all the valves, and you should have pressure on the brakes. This procedure is commonly referred to as a bench bleed.
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    Not Helpful 17 Helpful 2
  • Question
    Have a 1997 Grand Am. I replaced the master cylinder and power booster, no brakes still. What did I leave out?
    Community Answer
    There may be air in the brake line. Try bleeding the line.
    Thanks! We're glad this was helpful.
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    Not Helpful 9 Helpful 11
  • Question
    Why should a master cylinder be bled on the vehicle after installation?
    Community Answer
    After you change your brake/clutch master cylinder kit or replace the master cylinder assembly, you must bleed air/fluid to prevent 'trapped air' in the system after installing. There would be an 'air pocket' in the line. You have to pump many times by quickly opening and closing the bleed valve so as not to let air come back into the system.
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      Tips

      • If the new part doesn't have a reservoir installed, you will need to reuse the old one. Drain as much of the fluid as you can. If you want to clean any brake parts, use only denatured alcohol or brake cleaner. Petroleum based cleaners or water will wreck the seals.
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      • Buy new parts whenever possible. Rebuilt ones have a higher failure rate.
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      Warnings

      • If there is any doubt as to the safety and reliability of the braking system, do not put the vehicle back into service. Call a professional. This is cheaper and much less consequential than a crash you could have prevented.
        Thanks
        Helpful 7 Not Helpful 4
      • Do not use oils to clean anything that brake fluid is supposed to go into. This will destroy the seals.
        Thanks
        Helpful 6 Not Helpful 4
      • Do not reuse brake fluid that has been bled or poured out of the braking system. This will contaminate your new parts and damage them.
        Thanks
        Helpful 5 Not Helpful 6
      • Finally, it you have a sophisticated brake system such as ABS, EBD, or BA brakes, it may not be possible for you to avoid getting air into actuator. If you have this type of brake, you should have a professional bleed the brakes rather than doing it yourself.
        Thanks
        Helpful 4 Not Helpful 7


      • Brake fluid is very corrosive to paint and many plastics, even watch crystals. If you spill it on paint, wash it off immediately. [8]
        Thanks
        Helpful 1 Not Helpful 1
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      Expert Interview

      Thanks for reading our article! If you’d like to learn more about car maintenance and repair, check out our in-depth interview with Evan Ernst .

      References

      1. ↑ http://www.classicperform.com/TechBook/BrakeBleed.htm
      2. ↑ http://www.misterfixit.com/brakbld1.htm
      3. ↑ http://www.vw-resource.com/bench_bleed.html
      4. ↑ https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder
      5. ↑ https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system
      6. ↑ https://www.dummies.com/home-garden/car-repair/how-to-safely-jack-up-your-vehicle/
      7. ↑ https://www.familyhandyman.com/automotive/tires/car-repair-car-jack-safety/
      8. ↑ https://www.poison.org/articles/2016-sep/brake-fluid
      9. https://www.youtube.com/watch?v=Uz4OwuUGVOA&vl=en
      1. https://www.youtube.com/watch?v=n1NvtUwfRJc

      About This Article

      Article Summary X

      To bleed a master cylinder, start by lifting up the car with a jack so you can work underneath it. Next, have a helper slowly pump the brake pedal several times before keeping it held down. While the brake pedal is down, detach the hoses from the brakes to the master cylinder and let the brake fluid drain into a container. Finally, reattach the hoses so that air doesn’t get in and have your helper release the brake pedal. For tips on how to bleed a master cylinder using a bleed kit, read on!

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