After rebuilding or replacing the master cylinder on your ATV, you will need to bleed it separately to get all of the air out. The same applies if you bleed your brakes and accidentally let the brake fluid reservoir run dry, allowing huge amounts of air to enter the system.
In this post, we will have a look at 6 alternative ways of doing this procedure.
Bleed the ATV master cylinder, step-by-step
With this procedure, you bleed the master cylinder when it is still attached to the ATV.
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 common 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.
This is how you do it:
- Open the brake fluid reservoir lid and diaphragm. Be careful, the bolts strip easily.
- Pour new brake fluid into the reservoir so that it’s at the maximum level mark.
- Place plenty of rags to soak up the brake fluid being bled.
- Pull back the rubber cap covering the banjo bolt.
- Open the banjo bolt using the wrench. Remove the bolt completely from the cylinder.
- Use your finger to cover the hole where the banjo bolt was sitting. Remember to use gloves.
- Use your finger as a “valve” to let brake fluid out, but prevent air from getting back in when you pump the brakes.
- 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.
- When at about 80% of total travel and all the time when releasing the lever to do another pump, you should keep your finger firmly covering the hole. It is essential that your best to not let any air back into the hole at this stage.
- Continue this process for a few pumps. Add more fluid before it runs low.
- Repeat until there is no more air coming 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.
- Grab the banjo bolt with your free hand and get ready to thread it in the hole as effective as you can.
- Remove the finger and thread the bolt all the way.
- Tighten according to factory spec, it should be snug.
- Top off the reservoir and reinstall the reservoir cover.
- Bleed the brakes as normal 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 that’s left back into the master cylinder.
Alternative method – using the banjo bolt as a bleed valve
A variation of the above methods is when you instead of controlling the stream of fluid with your finger, you use the banjo bolt as a bleed valve.
Step 1 to 4 is identical 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 brake fluid starts sputtering out.
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.
Yet another alternative method – directing 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.
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 leed the brakes as normal.
Bench bleeding an ATV master cylinder
The expression “bench bleeding” come from when you remove 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.
Its actually faster and easier to bleed it when still attached on 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 with 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 upwards before you then fasten 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.
The same tools as the first method. In addition, you need the proper tools to remove the master cylinder from the handlebar. This varies between the brands and models of ATVs.
This is how you 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.
- Pull back the rubber cap covering the banjo bolt.
- Place a rag underneath to soak any spilled brake fluid.
- Open the banjo bolt using the wrench.
- Then proceed to remove the master cylinder assembly from the bike and moving it to your workbench.
- Secure the unit in a vice, but do not over tighten. The material cracks easily.
- Bleed the master cylinder as described in step 6 to 11 from the first method in this post.
- When you are done with step 11, top off the reservoir with new brake fluid and reinstall the cover.
- After reinstalling the cover, rotate the assembly so that the outlet port is facing upwards and move it over to the bike.
- Install the banjo bolt with the outlet port still facing upwards.
- Then reassemble the master cylinder to the handlebar.
- Finish off by bleeding the brakes as you normally would.
Bleed the master cylinder with a hand pump tool
If you’re not having any success bleeding the master cylinder just by pumping, you can try using a hand pump brake bleeding tool.
Depending on what model you get, these can be used for both pressing brake fluid through the master cylinder, or sucking brake fluid down by utilizing 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 there are no more bubbles appearing into 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 its own master cylinder. To bleed it properly, it is best to remove it and properly bench bleed it.
- 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.
- 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.
- Open the lid of the reservoir and top off with brake fluid.
- 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 seals on the piston inside the cylinder.
- Continue pumping until there are no more air bubbles coming out of the brake line, just a steady stream of brake fluid.
- Alternatively, you can direct the free end of brake line into a separate container. Then you need to 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 completely flush the system, replacing the old brake fluid with new.
- Keep the rubber hose submerged in brake fluid when you are done pumping. Clamp down the rubber hose near the caliper fitting to not let any air back into the system. Now you can safely take the hose out of the reservoir and reinstall the cover.
- Reinstall the assembly on the ATV, remove the clamp from the rubber hose and bleed the brakes normally.
Why can’t you just bleed the brakes the standard way to bleed the master cylinder?
After getting such a large amount of air into the master cylinder, you will likely have a hard time getting it all out with just the manual ways of pump-bleeding. That is if you are able to 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.
In addition, 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 with just adding fluid and bleeding as normal. But you will likely spend less time and get a better overall result if you bleed the master cylinder properly right away.