How to Bleed an ATV Master Cylinder (Front and Rear Brakes)

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After rebuilding or replacing the master cylinder on your ATV, you must bleed it separately to get all the air out. The same applies if you bleed your brakes and accidentally allow the brake fluid reservoir to run dry, allowing vast amounts of air to enter the system.

This post will look at six alternative ways of doing this procedure.

How to Bleed an ATV Master Cylinder: Step-By-Step

This Is my go-to method whenever I need to bleed the master cylinder on an ATV. With this procedure, you bleed the master cylinder when it is still attached to the ATV.

Tools Required

To perform this job, you need almost no tools. All you need is:

  • A screwdriver or hex wrench to open the brake fluid reservoir lid.
  • A wrench that fits the banjo bolt.
  • Brake-fluid according to manufacturer specs. The quality should be listed on the brake fluid reservoir lid or in your manual. Dot 4 is typical on ATVs.
  • A few rags to prevent spilling brake fluid on the plastic or paint on your bike, as this will create permanent damage.
  • Latex gloves. Brake fluid is corrosive and may damage your skin.

How to Do It

1. Open the brake fluid reservoir lid and diaphragm. Be careful; the bolts strip easily.

2. Pour new brake fluid into the reservoir so that it’s at the maximum level mark.

3. Place plenty of rags to soak up the brake fluid being bled.

4. Pull back the rubber cap covering the banjo bolt.

remove rubber cap banjo bolt atv brake master cylinder
Here you can see the banjo bolt still connected to the brake fluid reservoir.

5. Open the banjo bolt using the wrench. Remove the bolt from the cylinder.

remove banjo bolt polaris atv
When removing the banjo bolt, most of the brake fluid remains inside the reservoir.

6. Use your finger to cover the hole where the banjo bolt sits. Remember to wear gloves.

7. Use your finger as a “valve” to let brake fluid out, but prevent air from entering when you pump the brakes.

bleed master cylinder finger method
Use your finger to cover the exposed outlet port.

8. Start pumping the brake gently. Slightly release the pressure with your finger covering the hole to allow fluid to escape. Keep pressing the lever until you reach about 80% of its total travel.

9. When you’ve reached about 80% of the total travel and are about to release the lever for another pump, keep your finger firmly covering the hole. At this stage, it’s crucial not to let any air back into the hole.

10. Continue this process for a few pumps. Add more fluid before it runs low.

11. Repeat until no more air comes out with the brake fluid. You should be able to feel when this happens with your finger. It’s also quite visible when the bubbles are no longer coming—the stream of fluid changes from sputtering to a solid stream. Keep the finger pressed firmly against the opening when you are done.

12. Grab the banjo bolt with your free hand, and get ready to thread it in the hole as effectively as possible.

13. Remove the finger and thread the bolt all the way.

banjo bolt master cylinder
Remove your finger and insert the banjo bolt as fast as you can.

14. Tighten according to factory spec; it should be snug.

15. Top off the reservoir and reinstall the reservoir cover.

16. Bleed the brakes as usual to remove the last bit of air. Either use the manual pump and release method or use a vacuum bleeder. Reverse bleeding is not recommended at this point, as this will only press the small amount of air left back into the master cylinder.

Using the Banjo Bolt as a Bleed Valve

A variation of the above methods is when you, instead of controlling the fluid stream with your finger, use the banjo bolt as a bleed valve.

Steps 1 to 4 are the same as above.

5. Then pump the brake lever as you do when manually bleed the brakes.
6. But instead of opening the bleed valve by the caliper, you open the banjo bolt until the brake fluid starts sputtering out.

bleed master cylinder atv banjo bolt
Use the banjo bolt as a bleed valve.

7. Tighten the bolt when the brake lever is about 80% engaged.
8. Re-apply pressure and open the banjo bolt again.
9. Continue until you get a sputter-free stream of brake fluid.
10. Then bleed the brakes normally. You should now be able to build proper pressure.

This method may be a bit messier, so make sure you use plenty of rags to keep your ATV protected.

Related: 12 reasons why your ATV brakes won’t build or hold pressure.

Directing Brake Fluid Back Into the Reservoir

You’re probably starting to realize there is almost no limit to how many ways you can bleed the brakes on an ATV. Here is another tip to try.

Instead of letting the brake fluid squirt out of the banjo bolt and down onto a rag, you can direct it back up into the reservoir. Insert a piece of clear tubing into the outlet port on the reservoir and put the other end of the tube down into the master cylinder.

bleed master cylinder tip
Use a piece of tube to direct the brake fluid back into the reservoir.

Make sure it is completely submerged at all times. Then start pumping.

Continue until you see only clear brake fluid inside the clear tube. When you are done, install the banjo bolt quickly so as little air as possible gets into the system. Top off the brake fluid reservoir and bleed the brakes as normal.

Bench Bleeding an ATV Master Cylinder

The expression “bench bleeding” comes from removing the master cylinder off a car and bleed it on a workbench.

On an ATV, you don’t actually have to physically remove the master cylinder from the bike to “bench bleed” it, It is just as accessible where it is sitting on the bike.

It’s actually faster and easier to bleed it when still attached to the bike, but you run a small risk of spilling brake fluid on the ATV. Brake fluid is corrosive and will damage paint and plastic.

Another benefit of bleeding the reservoir on the bench is that you are free to rotate it so that the outlet port can be facing upwards when you are done bleeding.

This way, you can better prevent air from getting back into the system during installation. The banjo bolt can be installed with the port still facing upward before fastening the master cylinder to the handlebar again.

Ultimately it is up to you whether you want to bleed the master cylinder when still attached to the handlebar or if you want to remove the whole assembly and perform the job over at your workbench.

Tools Required

The same tools as the first method. Also, you need the proper tools to remove the master cylinder from the handlebar. This varies between the brands and models of ATVs.

How to Do It

The procedure is pretty much identical with bleeding the master cylinder when still attached to the bike. But before you begin, you need to remove the whole assembly from the ATV.

  1. Pull back the rubber cap covering the banjo bolt.
  2. Place a rag underneath to soak any spilled brake fluid.
  3. Open the banjo bolt using the wrench.
  4. Then proceed to remove the master cylinder assembly from the bike and moving it to your workbench.
  5. Secure the unit in a vice, but do not overtighten. The material cracks easily.
  6. Bleed the master cylinder as described in steps 6 to 11 from the first method in this post.
  7. When you are done with step 11, top off the reservoir with new brake fluid and reinstall the cover.
  8. After reinstalling the cover, rotate the assembly so that the outlet port is facing upwards and move it over to the bike.
  9. Install the banjo bolt with the outlet port still facing upwards.
  10. Then reassemble the master cylinder to the handlebar.
  11. Finish off by bleeding the brakes as you normally would.

Bleeding the Master Cylinder With a Hand Pump Tool

If you’re struggling to bleed the master cylinder merely by pumping, consider using a hand pump brake bleeding tool.

Depending on what model you buy, these can be used for pressing brake fluid through the master cylinder or sucking brake fluid down by vacuum.

The tool comes with a rubber adapter that can be inserted into the outlet hole on the reservoir. Follow the tool directions and press brake fluid back into the master cylinder assembly until no more bubbles appear in the reservoir.

Alternatively, you can switch the tool over to vacuum and suck brake fluid down from the reservoir and into the too through the same outlet port. If this does not help, you likely have a bad master cylinder and need to rebuild it to get it to start pumping again.

How to Bleed the Rear Brake Master Cylinder

The rear brakes on many ATVs will have their own master cylinder. To bleed it properly, it is best to remove it and properly bench bleed it.

atv brakes rear brake master cylinder pedal
Removing the rear master cylinder will usually require some disassembling as it is hidden behind plastic covers.
  1. Remove the master cylinder assembly, including the hose that goes to the rear brake caliper. Use a rag so that you don’t spill brake fluid on paint or plastic.
  2. Move the whole assembly over to your workbench. Find a way to keep it stable, but be careful if you put it in a vise. The aluminum or plastic materials will break if you apply too much pressure.
  3. Open the lid of the reservoir and top it off with brake fluid.
  4. Put the free end of the brake line back into the reservoir and start pumping slowly. Only pump about 80% of the overall travel before you start over, or you may damage the piston seals inside the cylinder.
  5. Continue pumping until no more air bubbles are coming out of the brake line, just a steady stream of brake fluid.
  6. Alternatively, you can direct the free end of the brake line into a separate container. Please make sure you add more brake fluid into the reservoir when it is running low. As an upside, this way of doing the job will thoroughly flush the system, replacing the old brake fluid with new.
  7. Keep the rubber hose submerged in brake fluid when you are done pumping. Clamp down the rubber hose near the caliper fitting not to let air back into the system. Now you can safely take the hose out of the reservoir and reinstall the cover.
  8. Reinstall the assembly onto the ATV.
  9. Remove the clamp from the rubber hose and bleed the brakes as usual.

Related: ATV brakes won’t bleed; 9 possible causes.

Why Standard Brake Bleeding Methods Fail Bleeding the Master Cylinder

If you’ve got significant amounts of air in the master cylinder, you will likely have difficulty getting it all out with just the manual ways of pump-bleeding. That is if you can get it to start pumping fluid at all.

Inside the master cylinder, there are a lot of pockets and passages where air can get trapped. The flow of brake fluid alone will not be able to reach it all and get it out.

Also, you are fighting gravity. All of the air inside the system wants to go up and into the master cylinder.

Vacuum bleeding may work, but not always. Air may still be trapped within these passages, even after performing a proper vacuum bleed. But it is worth trying if you already own a vacuum bleeder or can borrow one.

You may be lucky and achieve a decent result by adding fluid and bleeding as usual. But you will likely spend less time and get a better overall effect if you bleed the master cylinder properly right away.

Wrapping Up

With multiple methods at your disposal, bleeding the master cylinder on your ATV is a manageable task that can greatly improve brake performance.

Whether you use a hand pump tool, bench bleeding, or employ the banjo bolt technique, each approach has its unique advantages.

Always remember regular maintenance, including yearly bleeding, can significantly enhance the longevity and responsiveness of your ATV’s brake system.

FAQ

How often should I bleed the ATV’s master cylinder?

Bleeding should be done at least once a year or when brake responsiveness decreases or feels spongy.

Can I bleed the master cylinder on my ATV by myself?

Yes, it’s possible to bleed an ATV’s master cylinder alone, although a second person could make the process easier.

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Haavard Krislok
Haavard Krislok
I'm an ATV and offroad-enthusiast, an engineer, a farmer, and an avid home-mechanic. I'm also the owner and editor of BoostATV.com. If you have any questions or suggestions regarding this article, please feel free to contact me.

Welcome to BoostATV

Hi, I’m Haavard, the guy behind Boost ATV.  I made this site to share what I have learned as an avid ATV owner and enthusiast. I hope it will help boost your ATV experience! Learn More