Shimano’s SM-RT800 [paid link] disc brakes are renowned for their performance, delivering powerful and precise stopping power for road cyclists. However, it’s not uncommon for riders to experience spongy braking, where the brake lever feels soft or lacks immediate response. This issue can detract from your overall cycling experience, but fortunately, there are clear steps to diagnose and fix it. In this guide, we will explore the causes of spongy brakes and provide actionable solutions to restore optimal braking performance.
Understanding the Causes of Spongy Shimano SM-RT800 Disc Brakes
Before jumping into troubleshooting and fixes, it’s important to understand the underlying causes of spongy brakes. The main factors contributing to this issue are:
1. Air in the Brake System
One of the most common causes of spongy brakes is air trapped within the hydraulic brake system. Air bubbles can prevent the brake fluid from transmitting sufficient force to the brake pads, leading to a soft, ineffective lever feel.
2. Low Brake Fluid Levels
Brake fluid is crucial for maintaining hydraulic pressure and the proper operation of your brakes. If your Shimano SM-RT800 [paid link] disc brakes are low on brake fluid, this can cause sponginess when you pull the brake lever.
3. Brake Pads and Rotor Wear
Worn-out brake pads or a misaligned rotor can result in inefficient braking. If the pads are too thin or the rotor surface is damaged, your brakes may feel weaker and more spongy, especially under heavy use.
4. Incorrect Lever Adjustment
If the brake lever is not adjusted correctly, it may cause an excessive lever throw, resulting in a spongy or weak braking feeling. Ensuring proper lever travel is essential for smooth, responsive braking.
How to Fix Spongy Shimano SM-RT800 Disc Brakes
Now that you know the potential causes of spongy brakes, let’s go over the solutions to improve braking performance.
1. Bleed the Brakes
Air in the hydraulic brake lines is one of the most common reasons for spongy brakes. Bleeding the brakes will remove any trapped air and restore the proper hydraulic pressure.
Tools Required:
- Shimano brake bleed kit [paid link]
- 7mm or 8mm open-end wrench
- Brake fluid (Shimano mineral oil [paid link])
- Syringe and bleed block
Steps for Bleeding the Shimano SM-RT800 Disc Brakes:
- Set Up the Bike: Secure your bike on a work stand and remove the wheel to access the brake caliper and lever.
- Remove the Brake Pads: Take out the brake pads to avoid contaminating them with brake fluid during the bleeding process.
- Attach the Syringe to the Brake Caliper: Fill the syringe with Shimano mineral oil [paid link], and connect it to the caliper’s bleed port.
- Open the Bleed Port: Using the 7mm or 8mm wrench, gently loosen the bleed port screw.
- Pump the Syringe: Slowly push brake fluid through the system to remove any trapped air. Keep an eye on the syringe to ensure it remains filled with fluid.
- Close the Bleed Port: Once you see a steady flow of brake fluid without air bubbles, close the bleed port tightly.
- Test the Brakes: Before reassembling the bike, gently squeeze the brake lever. It should feel firm and responsive, with no sponginess.
2. Top Up the Brake Fluid
Low brake fluid can also cause spongy brakes. If the fluid level [paid link] is low, you need to top it up with Shimano mineral oil [paid link] to maintain proper brake performance.
Steps for Topping Up Brake Fluid:
- Locate the Fluid Reservoir: On your Shimano SM-RT800 [paid link] brakes, the fluid reservoir is typically located near the brake lever.
- Remove the Reservoir Cap: Use a small screwdriver to remove the cap and expose the brake fluid reservoir.
- Check the Fluid Level [paid link]: If the fluid level [paid link] is low, add Shimano mineral oil [paid link] until it reaches the proper level indicated on the reservoir.
- Replace the Reservoir Cap: After topping up the fluid, secure the cap back onto the reservoir and wipe off any excess fluid to prevent contamination of the brake pads.
- Test the Brakes: After refilling, check the lever feel again. It should now feel firm and responsive.
3. Replace Worn Brake Pads or Misaligned Rotors
Brake pads wear over time and can lose their effectiveness. If the pads are worn too thin, it can cause inconsistent braking and a spongy feel. Additionally, misalignment of the rotor or contamination from oil and grease can lead to weak braking.
Steps for Replacing Brake Pads and Aligning the Rotor:
- Inspect the Brake Pads: Check the thickness of the brake pads. If they are worn down to the wear indicators, replace them.
- Install New Brake Pads: Remove the old brake pads and install the new ones according to the manufacturer’s instructions.
- Check the Rotor for Damage: Inspect the rotor for any warping, cracks, or excessive wear. If the rotor is damaged, it should be replaced.
- Align the Rotor: If the rotor is slightly misaligned, use a rotor tool to adjust it. Ensure the rotor is centered between the brake pads and rotates freely without rubbing.
- Test the Brakes: After replacing the pads and ensuring the rotor is aligned, test the brakes to ensure proper function.
4. Adjust Brake Lever Travel
Brake lever travel can also impact brake performance. If the lever feels too soft or spongy, adjusting the lever travel may solve the problem.
Steps for Adjusting Brake Lever Travel:
- Locate the Adjuster: On Shimano SM-RT800 disc brakes, the brake lever usually has a small dial or screw to adjust lever travel.
- Turn the Adjuster: Turn the adjuster clockwise to reduce lever travel and create a firmer feel. Conversely, turning it counterclockwise can increase the travel if needed.
- Test the Lever: After making adjustments, squeeze the brake lever to see if it feels firm and responsive. If necessary, make further adjustments.
When to Seek Professional Help
While most issues with spongy Shimano SM-RT800 disc brakes can be resolved with the above steps, there are cases where professional intervention is necessary. If you’re experiencing persistent issues after following the troubleshooting steps, or if you’re unsure about any part of the process, consider taking your bike to a certified mechanic. Some issues may require more advanced techniques or replacement of internal components.
By following these steps, you can eliminate spongy brakes and restore reliable stopping power to your Shimano SM-RT800 disc brake system. Regular maintenance, such as checking fluid levels, inspecting brake pads, and bleeding the brakes periodically, will help ensure that your brakes continue to perform at their best.