Bleeding the Brakes on a 2020 Honda CBR300R ABS

Shop parts for a 2020 Honda CBR300R ABS Motorcycle.

The 2020 Honda CBR300R ABS uses hydraulic disc brakes at both ends, and the ABS badge means an anti-lock unit is part of the hydraulic circuit. A normal bleed at the caliper bleeders is the right way to refresh old fluid and remove air from the lines and calipers. This guide walks through that job in a safe, orderly way, with the ABS-specific limits called out so you know when the work should go to a shop.

What This Job Covers on This CBR300R

Bleeding is not just for a spongy lever. It is also the process of flushing old fluid out of the front and rear hydraulic systems and replacing it with fresh fluid. On this bike, both brakes are hydraulic, so both get attention. The front brake is operated by the lever and the rear by the pedal, and each has its own master cylinder, line, and caliper. The ABS unit sits in the hydraulic path, but for routine fluid service and air removal at the calipers, a standard bleed through the caliper bleeders is generally used. If air has entered the ABS unit itself—for example, because the reservoir ran dry, or because the ABS unit or its lines were disconnected—the system may need a dealer-level procedure or diagnostic tool. Do not try to improvise an ABS bleed sequence at home; send that work to a shop.

When a Bleed Is Due

Brake fluid absorbs moisture over time, which lowers its boiling point and can lead to brake fade under hard use. That is why many riders refresh the fluid on a regular schedule even when the brakes feel fine. A bleed is also due when the lever or pedal feels soft or spongy, when the lever pulls closer to the grip than it used to, when the fluid in the reservoir looks dark or cloudy, or after any work that opened the hydraulic system. New lines, caliper service, or master cylinder work are common reasons air gets in and a bleed becomes necessary. If the brake feels long or inconsistent, stop riding and inspect the system before assuming a bleed will fix it.

Tools and Supplies for a Clean, Controlled Bleed

  • Fresh brake fluid of the type marked on the master cylinder reservoir cap, from a sealed container
  • A box-end wrench that fits the bleeder valve
  • A length of clear tubing that fits snugly over the bleeder
  • A catch bottle for old fluid
  • Rags or shop towels
  • A syringe or turkey baster for removing old fluid from the reservoir
  • A screwdriver or driver that fits the reservoir cap screws
  • Optional: a one-way check-valve bleeder or vacuum bleeder to make one-person work easier

Use only fluid from a sealed container, and do not reuse fluid that has been sitting open. Brake fluid damages paint and plastic on contact, so keep rags handy and wipe up spills immediately.

Preparing the Bike and Protecting the Finish

Support the bike securely upright on a stand so it cannot tip while you are working the lever or pedal. Turn the handlebars so the front reservoir sits level before you open it. Cover painted surfaces, plastics, and the instrument area with rags, because most brake fluids damage paint and plastic quickly. If fluid spills, wipe it up immediately and rinse the area with water. Work in a well-ventilated space, and keep the reservoir cap and diaphragm clean so dirt does not fall into the fluid.

Bleeding the Front Brake

  1. Open the front reservoir and use the syringe or turkey baster to remove most of the old fluid. Do not let the reservoir run dry.
  2. Refill the reservoir with fresh fluid of the type marked on the cap, leaving room for the diaphragm.
  3. Attach the clear tubing to the front caliper bleeder and route the other end into the catch bottle.
  4. Squeeze and hold the front lever.
  5. Open the bleeder about a quarter turn to let fluid and air out.
  6. Close the bleeder before releasing the lever.
  7. Release the lever, then repeat the squeeze-hold-open-close sequence until fresh, clean, bubble-free fluid comes through the tube.
  8. Keep topping off the reservoir throughout so it never runs low enough to draw air in.

If a front end has more than one caliper, bleed them one at a time and begin with the caliper farthest from the master cylinder. On this CBR300R, treat the front caliper as its own bleed point and do not rush the sequence; the close-before-release step is what prevents air from being pulled back into the system.

Bleeding the Rear Brake

  1. Check the rear reservoir and remove most of the old fluid without letting it run dry.
  2. Refill with fresh fluid of the type marked on the cap.
  3. Fit the clear tubing over the rear caliper bleeder and route it into the catch bottle.
  4. Press and hold the rear pedal.
  5. Open the bleeder about a quarter turn.
  6. Close the bleeder before releasing the pedal.
  7. Release the pedal and repeat until the fluid in the tube is clean and free of bubbles.
  8. Top off the rear reservoir as you go so air cannot enter through the master cylinder.

The rear brake is hydraulic on this model, so it needs the same fluid service as the front. Keep the reservoir from running dry, and keep the bleeder closed whenever the pedal is released.

Finishing, Testing, and Troubleshooting

When the fluid runs clean and bubble-free at both ends, tighten each bleeder snugly without overtightening. Fill each reservoir to the correct level marking, reinstall the diaphragm and cap, and clean any spilled fluid. Check for leaks at the bleeder and at the banjo fittings. The lever and pedal should feel firm and consistent, not spongy, and should not slowly sink under steady pressure. Spin each wheel to confirm the brakes are not dragging. Test the brakes at low speed in a safe area before riding normally.

If the lever or pedal will not firm up after several bleeding attempts, if it firms up only after pumping, if bubbles keep appearing, or if fluid leaks from the master cylinder or caliper, stop and diagnose the problem rather than continuing to bleed. Any situation where air may have entered the ABS unit is also a reason to take the bike to a shop. A proper bleed should leave both brakes firm, clean, and ready for normal street use.

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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.