How To Bleed Brakes On A Dirt Bike

A way to ensure your brakes perform optimally.

Chances are if you ride dirt bikes or operate any other wheeled powersports machine, you will eventually have to bleed your brakes. Brake bleeding in itself is not a difficult job, but there are a handful of tips and tricks that can make the process go more smoothly and result in better-performing brakes.

Bleeding brakes on a dirt bike.
Bleeding brakes is not only necessary when there’s air in the line, it’s also a normal maintenance task that should be performed periodically.Courtesy of ProX

Servicing brake systems by replacing worn components as needed and refreshing the brake fluid periodically is a critical maintenance task that is often overlooked. In this article, we’ll provide an overview of the brake bleeding process and share some tips that can make the job easier. We’ll even provide a few pointers that can make rebuilding your master cylinder go more smoothly.

Checking brake lines and banjo bolts for damage and leaks.
If you’re feeling sponginess in the lever or less braking power, be sure to check your brake lines and banjo bolts for damage and leaks.Courtesy of ProX

The necessity to bleed your brakes may crop up for several reasons. The telltale signs that your brakes may need to be bled are reduced braking power, inconsistent braking, and sponginess felt in the lever. If any of these symptoms exist, it is always a good idea to check the integrity of the system. Leaks can occur as a result of loose banjo bolts, damaged seals, and cracked lines.

To start, it is always a good idea to consult your make and model’s factory service manual for information specific to the task at hand. Within your service manual, you’ll find torque specs, detailed procedures, and fluid information that can be important to take into consideration when performing the job.

The correct type of brake fluid for your motorcycle should be noted both in your owner’s manual and on the master cylinder cap.
The correct type of brake fluid for your motorcycle should be noted both in your owner’s manual and on the master cylinder cap. Do not use anything other than the recommended type.Courtesy of ProX

Brake fluids used in automotive and powersports applications within the United States and many other parts of the world are divided into three classes. Specifications for the classes are outlined by the Department of Transportation (DOT). The commonly available fluid class options are DOT 3, DOT 4, and DOT 5.1. The majority of powersports applications utilize DOT 4 brake fluid. Usually, your bike’s brake fluid reservoir cover/cap on the front and rear master cylinder denotes which fluid is used. In general, mixing of brake fluid classes is not recommended because of compatibility reasons and differences in boiling points.

Wearing gloves while bleeding brakes on a dirt bike.
Brake fluid is a very harsh chemical; be sure to protect yourself and your bike.Courtesy of ProX

Unfortunately, brake fluid is a harsh chemical and should be treated as such. Both you and your motorcycle can be adversely affected by inadvertent contact with brake fluid. When servicing your brakes and bleeding your lines, be sure to take any necessary precautions and avoid skin and eye contact. Wear rubber gloves and safety glasses when bleeding the brakes. If the brake lever is pumped too quickly during the bleeding process, fluid can be shot into the air from the reservoir. Brake fluid can strip paint and cloud plastics, so be sure to protect your bike’s surfaces with a towel or rags to limit exposure.

Towel wrapped around master cylinder.
Make sure your master cylinder is as level as possible to avoid spilling fluid. Wrap a towel or rag around the master cylinder in case of an accident.Courtesy of ProX

Prepare to bleed your brakes by positioning the front or rear master cylinder and reservoir as close to level as possible. When working on the front brake, this often means turning the handlebar to one side or the other to achieve a level reservoir. Don’t forget to position towels or rags around the reservoir in case of a spill.

Removing the master cylinder cap.
Be sure to use the correct tool(s) to remove the master cylinder cap to avoid stripping bolts.Courtesy of ProX

Once situated, remove the reservoir cover or cap, depending on which system you’re working on. Reservoir cover screws can be prone to stripping, so proceed cautiously. Typically, flat-head Japanese Industrial Standard (JIS) screws are used on Japanese equipment. While a Phillips bit is close in profile, it is not the right bit for the job and will more easily strip the fastener. Pick up a set of JIS screwdrivers or bits; otherwise, the tip of a Phillips bit can be ground down in a pinch so that the bit fits the JIS profile more closely.

Master cylinder cap removed.
After the cover or cap has been removed, proceed to remove the diaphragm. The diaphragm may come off with the cap, so be careful not to drop or damage it.Courtesy of ProX

Transition to the caliper and remove any necessary covers or guards so that the bleeder valve is visible. The bleeder valve has a rubber cover that protects its flared nipple and should be easily identifiable. Size a clear piece of tubing so that it fits snugly over the bleeder valve nipple and has at least 6 inches of length, or enough length to drain into a catch bottle.

Plastic tubing used to bleed brakes on a dirt bike.
Remove the protective rubber cap from the bleeder valve and place a piece of clear plastic tubing over it. Make sure the plastic tubing is snug and completely covering the flared end. The other end of the tubing should be placed into a catch bottle.Courtesy of ProX

To begin the bleeding process, squeeze the brake lever (front) or push the pedal down (rear), then open the bleed valve a quarter to a half turn. A mixture of air and brake fluid should exit from the bleeder valve and be visible in the clear tube. Close the bleed valve. Next, slowly release the brake lever or pedal and allow it to remain at its original extended position for a few seconds before repeating the process.

Squeezing the brake lever and loosening the bleeder valve.
Squeeze the brake lever (front), then loosen the bleeder valve approximately a quarter turn. Once the lever compresses all the way in to the grip, close the bleed valve, then release the lever. Repeat this process as necessary to work fluid and air out of the system.Courtesy of ProX

Keep a close eye on fluid levels in the reservoir. If all the fluid in the reservoir is consumed and air is sucked in, the whole process will need to be restarted. Add fresh brake fluid to the reservoir as necessary throughout the bleeding process to ensure no air ever gets sucked into the system. When adding brake fluid, be very careful to ensure no contaminants such as dust or debris enter the reservoir. Any small particles that find their way into the brake system can cause the system to malfunction or prematurely wear.

Keeping the fluid level in the master cylinder with a plastic syringe.
Keep the fluid level in the master cylinder high enough to avoid sucking air throughout the bleeding process. Using a plastic syringe is a great way to avoid spilling fluid when filling.Courtesy of ProX

The bleeding procedure outlined should be repeated until the mixture exiting the bleeder nipple consists entirely of brake fluid. This may take a short or long period of time depending on where and how much air is in the system. Once the brakes have been properly bled, a noticeable improvement in the feel of the brake lever or pedal should be felt when the brakes are actuated.

Clear plastic tubing on dirt bike as the brakes are bled.
Keep an eye on the clear tubing and bleed until there is nothing but clean brake fluid with no air bubbles exiting the bleed valve.Courtesy of ProX

Upon completion of bleeding the brakes, ensure the fluid level in the reservoir is at the upper level of the fill line. Then, carefully install the diaphragm, cover/cap, and any other remaining hardware. Be sure to double-check that the bleeder valve is tight. Once everything has been reassembled, test the brakes one final time by rolling the bike forward and backward, and noting how responsive the brakes are. Assuming everything checks out, it should be test ridden.

Testing the brakes by rolling the bike back and forth.
Once everything is put back together and retightened, test the brakes in the garage by just rolling the bike back and forth to make sure they are working properly before riding it.Courtesy of ProX

Perform a test ride by progressively increasing the speed and brake load. Do this based on brake performance and as you gain confidence in the brake system. Always pick a suitable test location and be sure to leave yourself ample room in case something goes amiss. Before performing high-speed and load braking tests, stop the bike and check over the caliper, line, and master cylinder to ensure there are no leaks.

Master Cylinder Tips

If you continue to encounter problems with your brake system after bleeding or find worn-out components in your master cylinder, it’s time to rebuild. Master cylinder rebuild kits make it easy and affordable to get your brakes performing again. The aftermarket is a source for rebuild kits for a job like this. Here’s a few tips to keep in mind during a master cylinder rebuild to ensure your job goes as smoothly as possible.

  • Be sure to use a snap ring plier when removing the snap ring from the master cylinder. Attempting to extract the snap ring with anything else often leads to damaging the snap ring groove and surrounding geometry.
  • Once the master cylinder has been completely disassembled, be sure to clean the entire master cylinder before reassembly. When cleaning the master cylinder, only use brake fluid.
  • Check the master piston and cylinder bore for abnormal scratches. If deep scratches are present, the piston and cylinder should be replaced. Also, reference your service manual for any applicable inspection measurements that should be taken.
  • Upon reassembly, be sure to apply new brake fluid to the master piston and master cylinder bore.
  • When installing the boot, be sure that it fully seats.

Should you encounter leaks or operational issues with your brake caliper, calipers can also be rebuilt and resealed.