Bleeding the Brakes on a 2019 Honda CBR300R

Shop parts for a 2019 Honda CBR300R Motorcycle.

The 2019 Honda CBR300R uses hydraulic disc brakes at both ends, with the front lever and rear pedal working separate hydraulic circuits. Some examples of this model were sold with ABS, so check your bike before you begin; if it has an ABS badge or you can see an ABS unit, treat the system as ABS-equipped. This is not a linked-brake motorcycle, so you are not following a combined-brake bleed sequence. The procedure below covers a normal fluid flush and air removal at the calipers. If air has entered an ABS unit, stop and let a shop handle it.

Why and When This CBR300R Needs a Bleed

Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. That moisture lowers the fluid’s boiling point and can make the brakes fade under hard or repeated use, even if the bike feels fine during easy riding. A soft or spongy front lever, a lever that now pulls closer to the grip than it used to, a rear pedal that feels vague, or dark and cloudy fluid in the reservoir are all signs that fresh fluid and a proper bleed are due. The same applies after any work that opened the hydraulic system, such as new brake lines, caliper service, or master cylinder work.

Many riders refresh brake fluid on a regular schedule even without symptoms, because contaminated fluid is difficult to judge by feel alone. On a lightweight street bike like the CBR300R, a firm, consistent lever and pedal are easy to notice once they are restored. If the lever slowly sinks while you hold steady pressure, or if it firms up only after several quick pumps, air is likely in the system and the bike should not be ridden normally until it is bled.

Tools and Supplies to Have Ready

  • 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 clean catch bottle for old fluid
  • Rags or shop towels, plus water for rinsing spills
  • A syringe or turkey baster for removing old fluid from the reservoir
  • A screwdriver that fits the reservoir cap screws
  • Optional: a one-way check-valve bleeder or vacuum bleeder to make one-person work easier

Do not substitute an open-end wrench for the bleeder if you can avoid it; a box-end wrench grips the bleeder more securely and reduces the chance of rounding it. Keep the fresh fluid capped until you are ready to pour, and discard any fluid that has been open for a long time or has been contaminated.

Setting Up Safely Before Opening the Reservoir

Support the CBR300R securely upright on a stand so it cannot tip while you are working. Turn the handlebars so the front reservoir sits level before you remove the cap; if the reservoir is tilted, fluid can spill as soon as the cap comes off. Cover the painted surfaces, plastic bodywork, and the instrument area with rags, because most brake fluids damage paint and plastic on contact. If a spill does happen, wipe it up immediately and rinse the area with water rather than letting it sit.

Work in a well-ventilated area and keep the reservoir cap and diaphragm clean once they are off. Before opening anything, look at the fluid level and condition through the sight glass or by removing the cap carefully. If the reservoir is already very low, find out why before you start bleeding; a leak at a caliper, banjo fitting, or master cylinder must be fixed first.

Step-by-Step Bleeding Procedure

  1. With the bike supported and the reservoir level, remove the reservoir cap and diaphragm. Use the syringe or turkey baster to draw most of the old fluid out of the reservoir without letting the level drop so low that air is drawn into the master cylinder.
  2. Refill the reservoir with fresh fluid of the type marked on the cap. Leave enough airspace for the diaphragm to sit correctly, but keep the level high enough that you can bleed without running dry.
  3. Fit the box-end wrench over the front caliper bleeder, then push the clear tubing onto the bleeder and route the other end into the catch bottle. The tubing should fit snugly so air cannot be pulled in around it.
  4. Squeeze the front brake lever and hold it steady. While you hold pressure, open the bleeder about a quarter turn. Old fluid and any trapped air will move through the tube into the bottle.
  5. Close the bleeder before you release the lever. Releasing the lever with the bleeder open can draw air back into the caliper. Then release the lever slowly and check the reservoir level.
  6. Top off the reservoir with fresh fluid and repeat the squeeze-hold-open-close-release sequence. Continue until the fluid coming through the tube is clean and free of bubbles. Never let the reservoir run low enough to pull air into the master cylinder.
  7. Move to the rear brake and repeat the same process with the rear pedal: press and hold the pedal, open the rear caliper bleeder, let fluid and air out, close the bleeder, then release the pedal. Keep the rear reservoir topped off as you work.
  8. If your bike has more than one front caliper, bleed each one in turn. As a general rule, start with the caliper farthest from its master cylinder, then finish with the closer one. On this CBR300R, work through the front caliper and rear caliper as separate circuits.

Finishing Up and Checking for Leaks

When the fluid runs clean and bubble-free at both ends, tighten each bleeder snugly without overtightening it. Over-tightening can damage the bleeder seat or make it difficult to open next time. Fill the reservoir to the correct level marking, reinstall the diaphragm and cap, and wipe away any fluid that collected around the reservoir or calipers. Rinse any spilled fluid with water and dry the area.

Inspect the bleeder valves and the banjo fittings at the master cylinder and calipers for weeping or wetness. A small amount of fluid around the bleeder is normal during the job, but it should be dry after you finish. Check that the reservoir cap and diaphragm are seated correctly, and make sure no tools or rags were left near the controls.

Testing Before You Ride Normally

The front lever and rear pedal should feel firm and consistent, not spongy, and they should not slowly sink while you hold steady pressure. Spin each wheel by hand to confirm the brakes are not dragging; a wheel that stops quickly or feels tight may indicate an overfilled system, a misadjusted component, or a caliper issue. Test the brakes at low speed in a safe area, such as an empty parking lot, before riding in traffic. If either brake feels different from what you expect, stop and investigate rather than assuming it will improve.

Troubleshooting and When to See a Shop

If the lever or pedal will not firm up after several bleeding attempts, or if it firms up only after pumping, air is still entering or remaining in the system. Persistent bubbles at the bleeder, fluid leaking from the master cylinder or caliper, or a lever that slowly sinks under steady pressure all point to a problem that needs attention. Do not keep riding a bike with a soft brake in the hope that it will settle.

If your CBR300R is ABS-equipped and the reservoir ran dry, or if the ABS unit or its lines were disconnected, the standard caliper bleed may not remove air from the ABS unit itself. That situation may require a dealer-level procedure or diagnostic tool, so take the bike to a shop. The same advice applies if you are unsure whether air has entered an ABS circuit or if the brakes will not hold pressure after a careful bleed.

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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.