How To Bleed Mountain Bike Brakes?

Properly bled brakes are crucial for safety and performance when riding rugged mountain bike trails. Over time, air and contaminants in the brake fluid can cause brakes to feel soft or spongy. Bleeding brakes flushes the system with fresh fluid to restore power and responsiveness.

While not overly complicated, bleeding brakes do require some specific tools and supplies. With the right preparation and by following key steps, mountain bikers can bleed their own brakes at home. This comprehensive DIY guide will walk through everything needed to successfully bleed Mountain Bike brakes.

Features and Benefits of Bleeding Brakes

• Restores brake power and responsiveness by flushing old fluid and air bubbles
• Extends brake system lifespan by preventing contamination buildup
• Essential routine maintenance for the optimal braking function
• DIY bleeding saves on bike shop repair costs
• Improved modulation for better brake ‘feel’ and control

When to Bleed Your Brakes

• Brakes feel soft or spongy
• Brakes lack power and need to be pulled too far
• Brake levers can be pulled all the way to the handlebars
• Braking causes squishy sounds or noises
• Brake fluid looks dark or contaminated

Bleeding the brakes at least annually is recommended, even if no issues are noticed, as part of regular bike maintenance. More frequent bleeding may be required depending on riding frequency, conditions, and issues noticed.

Tools and Materials Needed

• Brake bleed kit
• Mineral oil or DOT brake fluid
• Rubber gloves
• Clean rags
• Isopropyl alcohol
• Syringe or turkey baster
• Hex wrenches
• Torque wrench
• Bleed blocks
• Funnel and hose
• Catch container

A bike-specific bleed kit contains fittings needed to connect the brake lever and caliper during bleeding. Have plenty of clean rags, isopropyl alcohol for cleaning, hex wrenches suited to brake bolts, and the appropriate mineral oil or DOT brake fluid for your brakes.

See Also:
How To Adjust Mountain Bike Brakes?

How To Adjust Rear Derailleur On Mountain Bike?

Step-by-Step DIY Bleeding Process

Follow these key steps to properly bleed hydraulic disc brakes at home:

1. Prepare the Work Area

• Find a clean, dry, and well-lit work area where you have ample room to work comfortably. This can be a workbench or table.
• Lay down cardboard, rags, or trays to protect the surface from any brake fluid spills.
• Have some paper towels or a spray bottle of isopropyl alcohol handy to quickly clean up any minor spills.
• Ensure you have easy access to an electrical outlet if needed.
• Gather all the necessary tools and supplies and lay them out neatly in the work area within easy reach. Double check you have everything required.

2. Clean Brake Parts

• Use isopropyl alcohol and clean lint-free rags to thoroughly clean the brake calipers and levers. Pay extra attention to the bleed ports and fittings.
• Carefully spray or apply isopropyl alcohol and wipe down the caliper piston, caliper interior, lever barrel adjuster, and all bleed fittings.
• The goal is to remove any debris, grease, or contaminants that could enter the brake system.
• After cleaning, inject a small amount of fresh mineral oil or brake fluid into the caliper bleed port using a bleed syringe. This will flush out any remaining contaminants. Capture old drained fluid in a container.

3. Install Bleed Blocks

• Locate and install plastic bleed blocks, and spacers, between the brake pads in each caliper.
• This pushes the pads apart so they don’t close and damage the rotors when pumping the brake lever during the bleed process.
• Rotate each wheel slowly to ensure proper bleed block installation and that the wheel spins freely.

4. Attach Bleed Kit Fittings

• Attach the bleed kit tubing securely to the inlet bleed ports on the brake lever and caliper. These are normally located near the hose connections.
• Make sure the fittings are tightly sealed to prevent air leaks. Be careful not to cross-thread.
• Position the brake lever and caliper so that the bleed ports sit higher than the tubing outlets. This allows air bubbles to flow up into the tubes.

5. Open Bleed Ports

• Open the bleed ports on both the brake caliper and lever by unscrewing the bleed screws.
• Attach a funnel and clear tube to the caliper bleed port to capture old fluid draining out. Have a container ready to collect the drained fluid.
• If the lever doesn’t have a bleed screw, loosen the hose connection instead to allow air and fluid to escape.

6. Pump and Flush the System

• Have a partner repeatedly pull and release the brake lever to pump fluid through the system while watching the drain tube.
• Continue pumping the lever smoothly until the drained fluid coming from the funnel runs clean and free of air bubbles.
• Monitor the fluid level in the drain container so it doesn’t overflow. Top up the syringe if running low.
• Communication is key between the person pumping the lever and operating the syringe.

7. Refill with Fresh Fluid

• When the drained fluid is bubble-free, close the bleed port on the brake lever and disconnect the lower syringe from the caliper.
• Pour fresh, clean mineral oil or brake fluid into the caliper bleed syringe.
• Slowly depress the syringe plunger to fill the system from the caliper up through the lines to the lever with new fluid.

8. Bleed the Lever

• Reconnect the syringe and tubing to the brake lever bleed port.
• Again have your partner pump the brake lever while you operate the syringe, flushing new fluid through the lever until it runs clean.
• Monitor fluid levels and continue refilling the syringe as needed until the system is filled with fresh fluid.

9. Seal the System

• With both bleed ports and syringes connected and open, have your partner pull and hold the brake lever.
• While the lever is held, close the bleed ports by tightening the bleed screws with a hex wrench.
• Disconnect the syringes and tubing. The system should now be sealed and bled.

10. Check for Leaks and Proper Function

• Carefully inspect for any fluid leaks around the fittings and bleed ports. Tighten fittings if there is any seepage.
• Wipe off any brake fluid on the paint, rotors, etc.
• Spin each wheel to ensure no brake drag.
• Pull each brake lever firmly to test for proper pressure and responsiveness.
• If any issues are found, repeat the bleed process to fix them.

Tips for Bleeding Mountain Bike Brakes

• Go slow and be patient during the bleeding process to do it properly. Rushing can lead to air bubbles remaining in the system.
• Try to keep brake fluid off painted surfaces. Brake fluid will strip paint. Wipe up spills immediately.
• Always use fresh, clean DOT brake fluid or mineral oil. Never reuse old fluid.
• Monitor fluid levels in the syringe/container during bleeding. Refill syringe as needed.
• Turn the brake system adjuster barrels out before bleeding if the brakes feel overly tight.
• Consider using a one-way bleed valve for simpler, cleaner bleeds.
• Check brake pad thickness before bleeding. Replace pads if worn out.
• Bed in brake pads after bleeding by doing 20-30 hard stops. Helps establish proper pad-rotor contact.

Common Bleeding Mistakes to Avoid

• Letting air bubbles and contaminants get sucked back into the system. All parts must be air tight.
• Braking hard with bleed blocks still installed. This will badly damage pads and rotors.
• Not cleaning parts thoroughly before bleeding. Debris causes contamination.
• Running out of fluid mid-bleed and pulling in air bubbles.
• Not testing brakes cautiously before riding after bleeding.

FAQs on Bleeding Mountain Bike Brakes

How do I know when my brakes need bleeding?

Signs brakes need bleeding include soft or spongy levers, lack of power and modulation, noises when braking, and visible contaminants in the fluid. Bleed annually, even if there are no issues.

Can I bleed brakes myself, or should I go to a bike shop?

Home mechanics can absolutely bleed their own brakes with some patience and the right tools. A bike shop may be easier if you prefer.

What type of fluid do mountain bike brakes use?

Most use mineral oil, though some use DOT fluid. Check manufacturer specs to ensure using the recommended fluid type.

Can I reuse old brake fluid when bleeding brakes?

No, the old fluid should always be discarded. Only use fresh, clean fluid when bleeding to prevent brake system contamination.

Final Word

Bleeding mountain bike brakes is a crucial maintenance skill for DIY mountain bikers to master. Following the step-by-step guide above and utilizing the tips provided will set you up for success. 

Take it slow, be meticulous, and your brakes will come out feeling crisp and powerful.