Disc brakes on mountain bikes operate using hydraulic pressure that’s transmitted through brake lines when you squeeze the hand lever. Bleeding eliminates air that gets into the system when you service the brakes. Disc brakes are manufactured in a variety of styles and the process is relatively similar with each. An experienced mechanic can bleed the front and rear brakes on a bike in less than an hour. The average DIY bike mechanic can bleed disc brakes on a mountain bike using a disc brake bleeding kit that includes a syringe, pad spacers and DOT brake fluid.
Support the bike upright on a stand or on wood blocks. Chock the front wheel straight using a wheel stand or wood blocks.
Bleed the brakes one at a time, staring with the front brake. Loosen the bar clamps at the front brake reservoir at the handlebars using the appropriate screwdriver or Allen key. Position the reservoir so it is level and not tilted forward or back and tighten the clamp.
Secure plastic lunch bags around the base of the reservoir with rubber bands to protect the bars and grips from brake fluid. Loosen the reservoir cap bolts and remove the reservoir cap. Remove the rubber plunger from inside the reservoir and set it aside.
Remove the bolts that hold the front caliper to the fork using a metric wrench or Allen key. Slide the caliper off the disc rotor. Hold it in one hand and remove the retainer clip at the end of the brake pad bolt using a screwdriver. Loosen and remove the bolt using the screwdriver.
Push the brake pads out of the caliper by hand. Insert the pad spacer into the slot where the pads were removed to prevent the caliper pistons from popping out when you bleed the brake.
Attach a metric wrench on the bleeder fitting at the caliper. Hold the caliper over a glass jar and open the fitting 1/4-turn counterclockwise. Allow any fluid in the line to drain into the jar.
Attach the provided filler tube to the syringe. Refer to the instructions that came with the bleeding kit and draw the correct amount of DOT fluid into the syringe. Clean the residual fluid off the end of the tube with a rag.
Bleeding the Brakes
Push the end of the filler tube securely onto the caliper bleeder fitting by hand. Inject brake fluid into the line slowly as you observe the fluid level in the reservoir. Fill the reservoir to the top and close the bleeder fitting using the wrench.
Detach the filler tube from the fitting. Reattach the reservoir cap and tighten the cap bolts. Do not reinstall the rubber plunger into the reservoir at this point.
Hold the caliper over the glass jar. Open the bleeder fitting 1/4-turn and squeeze the front brake lever several times until fluid begins to flow into the jar. Tighten the bleeder fitting.
Refill the syringe with the correct amount of DOT fluid. Connect the filler tube to the bleeder fitting. Open the fitting 1/4-turn and inject fluid slowly into the line until it begins to back up in the filler tube. Close the bleeder fitting.
Remove the reservoir cap. Install the rubber plunger into the reservoir and reattach the cap securely. Push the spacers out of the caliper by hand and reinstall the brake pads using the saved bolt and retainer clip. Mount the caliper on the fork. Bleed the rear brake on the mountain bike using the same procedure.