Many mountain bikes use disc brakes, rather than traditional caliper brakes. Traditional brakes mount at the top of the frame and grasp the rim of the wheel. Disc brakes, however, mount close to the wheel axle and grip a smaller disc of metal mounted to the side of the axle. When the cyclist squeezes the brake lever it drives hydraulic fluid into the calipers, closing pistons that press brake pads against the disc. For disc brakes to work properly, you must fill them with hydraulic fluid and remove any air in the system.
Step 1
Position your bike in the repair stand so the hydraulic tubing is vertical from the brake caliper to the hydraulic fluid reservoir on the handlebar brake lever.
Step 2
Loosen the clamp on the brake lever and rotate it on the handlebar until the cover of the hydraulic fluid reservoir faces upward.
Step 3
Remove the wheel from the bike. Remove the bolt that holds the brake pads in place with an Allen wrench. Slide the brake pads out of the calipers. Place a 10 mm Allen wrench or brake block in the calipers to keep the caliper pistons open within the brake housing.
Step 4
Place the length of clear plastic tubing onto the end of the bleed valve on the caliper housing. Slip the small plastic bag onto the end of the tubing. Secure the bag to the tubing with the rubber band.
Step 5
Wipe the hydraulic fluid reservoir cover and brake lever with a clean cloth to remove any dirt or grime. Remove the hydraulic fluid reservoir cover and the bladder inside the reservoir. Place the bladder and the cover in the plastic container for safekeeping.
Step 6
Pour enough hydraulic fluid into the reservoir to fill it. Loosen the hydraulic fluid bleed valve on the caliper housing with the adjustable wrench. Pump the brake lever repeatedly to move hydraulic fluid into the brake system.
Step 7
Examine the hydraulic fluid in the reservoir for bubbles coming from the hydraulic tubing. Watch the plastic bag for hydraulic fluid to begin flowing from the bleed valve. Refill the reservoir periodically. Tap on the hydraulic tubing lightly to bring air bubbles up to the reservoir.
Step 8
Take note of the brake lever becoming stiffer and harder to squeeze as the system fills with hydraulic fluid. Tighten the bleed valve as air bubbles stop appearing in the hydraulic fluid reservoir and fluid runs freely into the bag.
Step 9
Hold the brake lever closed. Open the bleed valve one last time for about a second to remove any final air bubbles. Tighten the bleed valve and let go of the brake lever.
Step 10
Squeeze the brake lever to test the brakes. The brakes should feel stiff and offer resistance. If the brakes feel soft or mushy, bleed the line of any remaining air.
Step 11
Fill the hydraulic fluid reservoir to full capacity. Replace the bladder and reservoir cover. Remove the plastic tubing from the bleed valve. Wipe down the reservoir and bleed valve with a clean cloth to remove any excess hydraulic fluid.
Step 12
Remove the brake block or Allen wrench from between the caliper pistons. Reinsert and secure the brake pads. Reposition and tighten the brake lever on the handlebar. Replace the wheel.
Things You'll Need
- Bicycle stand
- Manufacturer-approved hydraulic fluid
- Small adjustable wrench
- Allen wrench set
- 10 mm Allen wrench or brake block
- Small plastic bag
- Plastic tubing
- Rubber band
- Clean cloth
- Small plastic container
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