How to Bleed GRX ST‑RX820 Di2 (future) Brake Sets at Home

Posted on
in Uncategorized

Bleeding the Shimano GRX ST‑RX820 Di2 (future) brake sets is an essential part of maintaining optimal braking performance, especially after prolonged use or when air enters the hydraulic system. This advanced gravel-specific Di2 system combines electronic shifting with precise hydraulic braking. Keeping the hydraulic fluid clean and air-free ensures that braking remains sharp and consistent across mixed terrain. This guide provides a complete walkthrough for home mechanics looking to bleed their GRX ST‑RX820 Di2 brakes accurately and safely.

Tools Required

  • 2.5mm and 4mm Allen wrenches [paid link]
  • Shimano TL-BR002 funnel or equivalent
  • Shimano mineral oil [paid link] (do not use DOT fluid)
  • Bleed block compatible with Shimano flat-mount calipers
  • 7mm open-end wrench
  • Syringe with tubing (Shimano TL-BR001 or similar)
  • Isopropyl alcohol and lint-free cloth
  • Plastic catch container
  • Nitrile gloves
  • Torque wrench

Preparation and Safety

Before beginning, place the bike in a stable stand with both wheels installed to maintain realistic hose routing. Remove the wheel only if necessary to access the caliper. Ensure the Di2 system is powered off and shift the rear derailleur into the smallest cog to avoid drivetrain interference.

Clean the lever and caliper area using isopropyl alcohol to prevent contamination of the brake fluid. Wear gloves to protect your skin from mineral oil and maintain cleanliness throughout the process.

Installing the Bleed Block

The bleed block replaces the brake pads during the bleeding process and keeps the pistons from moving.

  1. Remove the wheel and brake pads from the caliper.
  2. Insert the bleed block and secure it with the pad retention bolt or pin.
  3. Reinstall the caliper cover or keep it open for access to the bleed nipple.

Proper positioning of the bleed block ensures the hydraulic pistons remain stationary, preventing damage or misalignment.

Attaching the Funnel and Bleed Syringe

On the GRX ST‑RX820 Di2 levers, the bleed port is located beneath a small screw cover at the top of the lever body.

  1. Rotate the handlebar so the bleed port is level.
  2. Use a 2.5mm Allen wrench to remove the bleed port screw.
  3. Thread the Shimano bleed funnel into the lever body.
  4. Fill the funnel halfway with fresh Shimano mineral oil.
  5. At the caliper, connect the syringe tubing to the bleed nipple using a snug fit.
  6. Fill the syringe with a small amount of oil and remove air bubbles before connection.

Bleeding the System

Bleeding is performed using a top-to-bottom method, with oil flowing from the lever down to the caliper via gravity and slight pressure.

  1. Open the bleed nipple on the caliper using a 7mm wrench.
  2. Gently push the syringe to force oil upward through the caliper and hose toward the funnel.
  3. Observe the funnel for bubbles rising to the surface.
  4. As bubbles stop appearing, lightly tap the caliper and hose to dislodge trapped air.
  5. Continue pushing oil until clean, bubble-free fluid appears at the funnel.

Take care not to overpressurize the system, as excessive force can dislodge seals or cause leaks.

Closing the System

After completing the fluid exchange, it’s important to seal the system properly.

  1. Tighten the bleed nipple while maintaining pressure on the syringe to prevent air ingress.
  2. Remove the syringe and wipe the area with isopropyl alcohol.
  3. Remove the funnel and reinstall the bleed screw with the specified torque.
  4. Clean any excess oil from the lever and caliper thoroughly.

Use a torque wrench to avoid overtightening bleed screws, which could damage the aluminum threads.

Lever Feel and Pressure Check

Squeeze the lever repeatedly to verify brake pressure is consistent and firm. If the lever feels spongy or returns slowly, repeat the bleeding procedure. For Di2 levers, a crisp, direct engagement should be immediately noticeable.

Reinstalling Brake Pads and Wheel

Once pressure and feel are confirmed:

  1. Remove the bleed block and inspect the caliper.
  2. Reinstall the brake pads and secure the pad pin.
  3. Reinstall the wheel and center the caliper if needed.
  4. Spin the wheel and actuate the brake to ensure correct alignment and pad contact.

Caliper centering may require minor adjustments using a 4mm Allen wrench on the mounting bolts.

Troubleshooting Common Bleeding Issues

Several issues may arise during or after bleeding the GRX ST‑RX820 Di2 system:

  • Soft Lever Feel: Indicates air remains in the system. Repeat the bleeding process.
  • Oil Leakage at Lever or Caliper: Check O-rings and thread fittings. Ensure correct torque.
  • No Pressure Build-up: The funnel may have been introduced too late or air was trapped early in the process.

Always use genuine Shimano mineral oil and tools to avoid compatibility and sealing issues.

Customizing Brake Feel

Shimano Di2 levers, including the ST‑RX820, offer a degree of lever reach and bite point adjustment.

  • Reach Adjustment: Accessed via a small screw on the lever blade pivot, allowing for ergonomic tuning.
  • Bite Point Adjustment: While limited in range, it affects pad engagement distance slightly. Adjust with the inline screw if available.

Tailoring lever position ensures comfortable braking over long gravel rides and technical descents.

Compatibility Considerations

The GRX ST‑RX820 Di2 brake system is optimized for use with:

  • Shimano GRX BR‑RX820 calipers
  • Shimano SM-BH90 hoses
  • Shimano RT-MT800 or RT-CL800 rotors

Using non-compatible hoses, pads, or rotors can lead to inconsistent braking, excess noise, or premature wear.

Comparison With Similar Shimano Brake Systems

Compared to the previous GRX 810 mechanical or Di2 systems, the ST‑RX820 (future) introduces refined ergonomics, better hose routing, and improved bleed ports. When compared with Shimano’s Ultegra R8170 or 105 R7170 Di2 systems, the GRX prioritizes off-road ruggedness with a broader bite point and more aggressive pad compound options.

Its caliper and lever shapes are tuned for flared gravel bars and gloved operation, providing a more confident interface in mixed terrain.

Maintenance Frequency and Best Practices

To keep the GRX ST‑RX820 Di2 brakes operating flawlessly:

  • Perform a full bleed every 6 to 12 months, or after fluid contamination or crash damage.
  • Clean the lever and caliper area monthly to remove dust and grime.
  • Avoid squeezing the lever when the wheel or pads are removed.
  • Inspect hose condition during each bleed to prevent mid-ride failures.

Regular upkeep enhances brake longevity and maintains Shimano’s factory-level performance across all riding conditions.