Brakes that for some reason won’t build or hold pressure is a common issue people have with their ATVs. In this post, we’ll have a look at the most common causes why this happens, and how to fix it.

1. There is still air trapped in the system after replacing brake components (most likely cause)

This one is the most likely cause, even if you think you got all of the air out. No air bubbles coming out when you bleed as normal does not necessarily mean that there is no air left in the system.

Any time you replace or disassemble any of the brake components that are in direct contact with the brake fluid, air will enter the otherwise closed brake line system.

New brake lines are going to be full of air. This air needs to be bled out entirely before the brakes will start working as they should.

If the brakes feel mushy, there is more than likely still air inside, even if you think you got all of the air out.

Here are a few tips  to follow for successfully replacing brake components:

  • Set up all of the components where they should be, but don’t tighten anything before all of the components are where they should be.
  • Proceed by tightening all of the connections and brake line fittings, but do not yet tighten the banjo bolts down where the brake lines meet the brake calipers.
  • Fill up the brake fluid reservoir and start pumping. Add more fluid before it drops below the minimum mark. If you do this operation manually without a vacuum bleeder, expect to pump the brake a lot of times before you have got fluid through the system. Patience is vital for this part of the procedure.
  • Continue pumping and adding more fluid until you see fluid coming out down by the loose banjo bolts.
  • Now tighten the banjo bolts down by the brake calipers as well.
  • Proceed by bleeding the brakes as normal, one caliper at a time. Always begin with the caliper the furthest from the master cylinder.
  • Make sure all of the fittings are completely tight and not leaking. If you see any leaks, disassemble the leaking connection, clean and reinstall. All connections should be tightened snug but do not overtighten.

If you’re still having problems, try this:

  • Rotate the handlebar so that the master cylinder is at its highest.
  • Open the brake fluid reservoir cap and remove the rubber diaphragm.
atv rubber diaphragm
This is how the rubber diaphragm looks like.
  • Take a clean, flat small screwdriver and remove the little rubber plug at the bottom of the reservoir. Note that not all master cylinders have a rubber plug at the bottom. This trick only works for the ones that do.
  • Jiggle the brake lever to remove any air bubbles still left at the top of the brake line.
  • Reinstall the rubber plug and add more brake fluid if needed.

Another a popular method for ATV brake bleeding that usually will work is reverse bleeding. This post will tell you how to reverse bleed the brakes on your ATV.

2. Piston seals in the master cylinder have gone bad after beeing left dry for some time

The rubber piston seals in a brake system spend their life in a highly corrosive environment. If these seals are left dry for more than just a few days, they may develop knicks which make them useless.

atv master cylinder piston seal
Through a small hole at the bottom of the brake fluid reservoir, you can get a glimpse of the piston and the piston rubber seal.

This may happen if you’re waiting for parts to arrive or doing a major rebuild that leaves the master cylinder dry.

Try keeping the rubber seals soaked in brake fluid if you’re not able to finish the job in a few days. If they crack and go bad, you will need a rebuild kit.

Any time you are planning a major rebuild of an old ATV, or a bike that has been standing for some time, it’s a good idea to throw a rebuild kit into your cart along with the other parts you plan on replacing. Expect to pay $10 – $35 for a complete rebuild kit that consists of:

  • A piston
  • Piston spring
  • E-clip
  • Dust boot
  • Piston seals

3. The brake fluid has become dirty

If the brake fluid gets contaminated or dirty, the master cylinder won’t work as it should. The dirt will build up inside the small canals and compartments of the master cylinder housing, possibly preventing proper brake fluid flow.

Rubber gaskets and parts sliding over the dirt buildup will allow brake fluid to bypass where it shouldn’t, and the piston won’t be able to build pressure as it should.

To fix this issue, you need to:

•   Bleed all of the old brake fluid out. Just open the bleed valve and pump the brake lever until all of the fluid is out.

•   Disassemble and clean the inside of the master cylinder using brake cleaner and a small brush.

•   The dirt has likely worn the rubber O-rings and gaskets inside the master cylinder. Get a master cylinder replacement kit and replace these parts.

•   Reinstall the master cylinder and add new brake fluid according do factory spec (usually DOT4).

If you have any problems with bleeding the master cylinder after doing this cleanup job, have a look at this post where we look at several different ways of bleeding a master cylinder, or this post about ATV brakes that won’t bleed.

4. Brake lines are damaged or corroded

When we’re doing off-road riding, brake lines may get damaged from smashing into rocks and branches. A smashed brake line won’t alow brake fluid to flow as it should. Or, there may be corrosion buildup that will also prevent proper flow.

Reinforced brake lines that are damaged will not provide a rock hard brake feel as they should.

Inspect the brake lines all the way from the master cylinder and down to the caliper. If you see any signs of damage or corrosion, replace the damaged component with a new one and see if this fixes your issue.

atv brake line
You should be able to follow the brake lines from the calipers and up to the master cylinder.

5. Moisture getting into the master cylinder

If moisture gets into the brake system and the master cylinder, there will be a residue build up over time. This residue will prevent the rubber seals from closing entirely, letting brake fluid bypass when you brake.

To fix this issue, you have to remove all brake fluid, flush the brake system with plenty of new brake fluid, clean out any residue from the master cylinder, and get a master cylinder rebuild kit.

Make sure the brake fluid reservoir cap is sealing properly so the problem won’t come back. Get a new seal if it’s not included in the master cylinder rebuild kit.

6. Brake shoes not adjusted properly before bleeding brakes

The brake shoes on drum brakes need to be adjusted properly before you can successfully bleed the brakes. If you don’t, air may still be trapped, and you will not be able to build brake pressure.

Note that this only applies with drum brakes, not disc brakes.

Do this on all brake shoe adjusters, one at the time. Tighten the adjuster until the brake locks up. Then back off the adjuster three clicks and move on to the next adjuster. Repeat on all wheels that have drum brakes before bleeding the brakes as normal.

7. Brake fluid leak somewhere in the brake system

If your brake system leaks, you will likely have problems building proper pressure. The leak does not have to be big before you start noticing that something is not right.

On a dirty bike, a small leak may be just barely visible, without any brake fluid dripping to the ground. There is just a moist spot in the area where the leak is located.

Again, you need to inspect the brake lines from the master cylinder, and down to the brake calipers. Use a clean piece of paper and drag it along the brake line; this will make it easier to pick up any leaked brake fluid.

The leak may just as well be in the master cylinder itself, or any of the calipers. Again, use a piece of paper towel to find the leak.

Replace any damaged parts. If the leak is in the master cylinder or brake caliper, you will need a master cylinder rebuild kit.

If the leak is in any of the brake line connections, such as the banjo bolt up by the master cylinder, you may be able to stop the leak just by cleaning.

  • Open the brake fluid reservoir cover.
  • Use a syringe to suck out the brake fluid that’s in the reservoir.
syringe brake fluid oil
Syringes are great for removing hard to reach fluids, such as brake fluid.
  • Remove the banjo bolt.
open banjo bolt master cylinder
Use a rag to prevent brake fluid from ruining the paint on yout ATV. Brake fluid is corrosive.
  • Clean everything, including the crusher washer and the connecting surface properly using a nylon brush and some brake clean.
  • Reinstall and tighten according to factory spec.
  • Add fresh brake fluid.

8. No brake drums installed

If you are doing a brake rebuild and try building brake pressure before you install the brake drums, you will likely not succeed.

The drums must be installed when you try building brake pressure. If they are not, the plungers inside the brake cylinders will get pushed out too far, allowing air to get back into the system.

Install the drums, bleed the brakes, and see if that’s what was causing your issue.

9. The master cylinder does not have the capacity to feed  new brake lines after replacing them

You won’t always be able to push new brake fluid down empty new brake lines using just the method of pumping the master cylinder repeatedly.

If the level in the brake fluid reservoir does not seem to be dropping after 10-20 pumps on the brake lever, you will likely need to use another method for bleeding the brakes.

My favorite method to try in situations like this is using a simple vacuum bleeder to suck the new brake fluid down the brake lines. You can get at any auto supply store for 10-20 bucks.

I write about how to use one in this post that also covers a range of other methods you can use to bleed the brakes.

10. Bad rubber seal in the brake caliper

When you replace worn brake pads, you need to push the plunger in the brake caliper back to make room for new ones. If you are not careful, you may damage the rubber seal, causing a leak. The seal may also be bad just due to old age.

atv brake piston
Not all ATV brake calipers have visible rubber piston seals, like on this Polaris.

If you’re working on a clean, smooth surface, you should be able to spot a leak lin this area quite fast. If you’re working on dirt or gravel, you may end up refilling the brake fluid reservoir several times before you notice the problem.

If you damage the seal, there is no way around replacing it. Rebuild kits are cheap, and the job is relatively easy to do.

11. Air-locked master cylinder

Inside the master cylinder, there are many tight spots where small bubbles of air can get trapped. If you’ve got air in the master cylinder, you may have a hard time getting it to start building pressure.

Read this post about how to bleed the master cylinder properly. Doing so should get you started. Then you can proceed to bleed your brakes normally.

12. Air trapped in the second master cylinder (rear brakes)

Don’t forget that your ATV will have two master cylinders: One up on the handlebars for the front (and some times also the rear wheels), and one down by the foot brake for the rear wheels.

Depending on how the brake system on your bike is designed, you need to make sure both master cylinders are properly bled.