How to Bleed and Refill KOOTU Hydraulic Disc Brakes
Hydraulic disc brakes provide powerful, consistent braking performance and have become the standard choice for modern road bikes, gravel bikes, and mountain bikes. However, over time, air can enter the hydraulic system, causing the brake lever to feel soft or spongy and reducing braking efficiency.
If your brakes no longer feel as firm as they should, it may be time to perform a brake bleed and refill the hydraulic fluid.
In this guide, we'll explain when hydraulic brakes need bleeding, what tools you'll need, and how to properly bleed and refill your hydraulic disc brakes.
What Is Hydraulic Brake Bleeding?
Brake bleeding is the process of:
Removing trapped air from the hydraulic system
Replacing old brake fluid with fresh fluid
Restoring proper brake lever feel and braking performance
Unlike mechanical disc brakes, hydraulic brakes rely on fluid pressure to transfer force from the brake lever to the caliper. Even a small amount of air in the system can significantly affect braking performance.
Signs Your Hydraulic Brakes Need Bleeding
You may need to bleed your brakes if you notice any of the following symptoms:
Soft or Spongy Brake Lever
The brake lever feels less firm than usual when squeezed.
Excessive Lever Travel
The brake lever moves much farther toward the handlebar before engaging the brakes.
Reduced Braking Power
Your brakes no longer feel as powerful or responsive as before.
Recent Brake Service or Hose Replacement
Any time the hydraulic system has been opened, air may have entered the system.
Long-Term Storage
Bikes that have been stored for extended periods may occasionally develop air bubbles within the brake system.
Tools and Materials You'll Need
6mm Allen key
3mm Allen key
T15 star-shaped bit
M4 oiling syringe
IIIPRO mineral oil or Shimano mineral oil (do not use any other mineral oil)
M4 oiling syringe
Isopropyl alcohol or rubbing alcohol
Nitrile gloves
Paper towels or cotton swabs
Detailed Steps
First, remove the wheel set, loosen the brake pad bolts, insert the spacers, and tighten the brake pad bolts.
Use the syringe to draw up 40 ml of mineral oil, expel all air bubbles from the syringe, connect the oil syringe to the caliper, and then tighten it completely. The lubrication screw at the shifter position has already been removed. Next, install the oil reservoir, tighten the reservoir screw, open the reservoir, and adjust it to a horizontal position. The bleeding process is as follows: first, push the fluid up through the syringe. Do this as slowly as possible while monitoring the fluid level in the reservoir. Once all the fluid has been pushed through, squeeze the brake lever. Repeat this sequence two to three times, squeezing the brake lever at a rate of approximately once per second. Once there are no more air bubbles, remove the syringe and continue bleeding the system by squeezing the brake lever at a rate of once per second. Continue until no more air bubbles escape from the reservoir. Then close the reservoir, remove it, reinstall the bleed screw, and clean the shifter. Clean the caliper area, remove the caliper plugs, and reinstall the brake pads, pad bolts, and wheel hub. Finally, check the brake feel. If the brake lever operates very smoothly and provides firm stopping power when squeezed, the bleeding and fluid change have been successful.
Final Thoughts
Hydraulic disc brakes offer outstanding stopping power and control, but they depend on a properly maintained hydraulic system. If your brake lever feels soft, braking performance has decreased, or air has entered the system, a proper brake bleed can restore your brakes to peak performance.
By following the correct bleeding procedure and using the proper brake fluid, you can keep your road bike or gravel bike braking smoothly, safely, and reliably for every ride.
