Shimano’s SM-RT900 [paid link] disc brakes are renowned for their precision, reliability, and power. However, many cyclists experience an issue where their brakes feel spongy, compromising performance and safety. This sensation is often due to air in the hydraulic lines or improper brake setup. Fortunately, there are simple ways to restore your brakes to their optimal performance. In this guide, we’ll delve into the causes of spongy brakes in Shimano SM-RT900 [paid link] systems and how to fix them quickly and effectively.

Understanding the Causes of Spongy Shimano SM-RT900 Disc Brakes

Spongy brakes typically result from air or moisture entering the hydraulic braking system. This disrupts the pressure necessary for efficient braking, making the levers feel soft or unresponsive. Here are the common causes of sponginess in the Shimano SM-RT900 [paid link] system:

1. Air in the Hydraulic Lines
   The most common cause of spongy brakes is the presence of air in the hydraulic brake lines. When air is trapped in the system, it compresses when you pull the brake lever, causing the spongy feeling. This issue is typically a result of improper bleeding or an incomplete brake setup.
2. Contaminated Brake Fluid
   Over time, the hydraulic brake fluid can become contaminated with water or dirt, reducing its effectiveness. This can lead to a spongy feel as the fluid no longer maintains the required pressure in the system.
3. Worn Brake Pads
   While worn brake pads won’t directly cause sponginess in the brake lever, they can contribute to poor braking performance, making the brake lever feel soft as it fails to engage with the rotor effectively.
4. Damaged or Leaky Hydraulic Hoses
   Hydraulic brake hoses are crucial for maintaining the pressure in the brake system. Any damage or leaks can lead to a loss of pressure, resulting in sponginess. Inspect the hoses regularly for cracks, abrasions, or leaks.
5. Improper Brake Lever Setup
   If your brake levers aren’t adjusted correctly, they can cause sponginess, as the lever stroke may not properly engage the brake calipers.

How to Fix Spongy Shimano SM-RT900 Disc Brakes

To restore your Shimano SM-RT900 disc brakes to their peak performance, you’ll need to address the root causes of sponginess. Here’s a step-by-step guide to fixing the issue:

1. Bleed the Brakes

The first and most effective solution is to bleed the brakes and remove any air in the hydraulic system. Shimano provides a specific process for this, but generally, the steps involve:

• Tools Required:
  • Shimano Bleed Kit [paid link] (SM-BCR2)
  • Syringe and Tubing
  • Mineral Oil [paid link] (Shimano Hydraulic Fluid)
  • Bleed Block
  • 8mm Wrench
• Steps:
  1. Secure your bike in a stand to ensure it remains stable during the process.
  2. Remove the wheel to gain access to the brake caliper.
  3. Attach the syringe to the brake caliper and the other syringe to the brake lever port, filling both with brake fluid.
  4. Open the bleed valve on the caliper, allowing the fluid to flow into the system while pushing out any air.
  5. Close the bleed valve, then pump the brake lever to test for pressure.
  6. Repeat the process until the brake lever feels firm, indicating all air has been expelled.

2. Replace Contaminated Brake Fluid

If the fluid is contaminated with moisture or dirt, it must be replaced. After the bleeding process, check the brake fluid’s condition. If it appears cloudy or has particles in it, replace it with fresh Shimano mineral oil [paid link].

3. Check and Replace Brake Pads

Worn brake pads can exacerbate the feeling of sponginess. If your pads are below the recommended wear limit, replace them with new Shimano-compatible pads. Be sure to inspect the calipers and pistons for any signs of wear or damage as well.

• Steps for Replacing Pads:
  1. Remove the caliper from the mounting bracket.
  2. Take out the old brake pads and insert the new ones.
  3. Reinstall the caliper, ensuring that the pads align properly with the rotor.
  4. Reattach the wheel and test the brakes for responsiveness.

4. Inspect for Leaks in the Hydraulic System

Check your hydraulic hoses for any signs of leaks or damage. Leaks can significantly affect brake performance and lead to a spongy feel. If you notice any damage, replace the hose. It’s important to ensure all connections are tight and secure.

5. Adjust Brake Lever Reach and Position

Sometimes, adjusting the reach and position of the brake levers can eliminate the feeling of sponginess. Set the brake lever reach so that your hand can engage the lever comfortably without excessive force. This ensures the correct amount of pressure is applied, and the calipers function optimally.

6. Re-test and Fine-tune the Setup

After performing the necessary maintenance steps, test the brakes by riding the bike at slow speeds and applying the brakes to check for any remaining sponginess. Fine-tune the brake lever’s reach and the rotor alignment to ensure maximum braking efficiency.

Preventing Spongy Brakes in the Future

Once your Shimano SM-RT900 disc brakes are functioning properly, there are a few things you can do to prevent future issues with sponginess:

• Regularly Check Fluid Levels: Ensure that the brake fluid remains at the correct level [paid link] and is free of contaminants.
• Clean and Lubricate the Brake System: Keep your brakes clean and well-lubricated to avoid unnecessary wear on the components.
• Monitor Pad Wear: Regularly check your brake pads and replace them when necessary to maintain optimal braking power.
• Inspect the Hydraulic System: Periodically check for leaks or signs of wear in the hydraulic lines and connections.

By following these tips and regularly maintaining your Shimano SM-RT900 disc brakes, you can keep them in peak condition and avoid issues with sponginess, ensuring reliable braking power when you need it most.