Bleeding hydraulic disc brakes

Preparing the bleed workspace

Hydraulic fluid is unforgiving to paint and braking surfaces alike. Before you open any ports, find a clean, flat surface and lay down a heavy rag or cardboard. Remove the wheels and slide a bleed block into the caliper to keep the pistons from advancing. Removing the wheels is crucial because even a single drop of brake fluid on your braking surfaces can ruin them. If you are also managing other maintenance tasks, like e-bike battery care or cleaning the frame, finish those first to keep your workspace clear.

Gather your bleed kit, which should include the correct fluid for your specific system. Mineral oil and DOT fluid must never be mixed, as they will destroy the seals of an incompatible brake. “Safety first with DOT fluid,” my local mechanic always says, referring to its corrosive nature, so keep some rubbing alcohol nearby to quickly neutralise any accidental spills on the frame. Keep a clean paper towel in your hand at all times to wipe away excess fluid immediately.

Connecting the syringes

Draw mineral oil or DOT fluid — depending on your brake brand — into your syringes. You will need to purge any air bubbles from the syringes by holding them upright, tapping the sides, and pushing the air out through the top. Thread the first syringe into the lever bleed port. If you are used to packing a bike for travel, you might recognise the delicate threads on these ports; they strip easily, so turn them with care.

Connect the second syringe, half-filled with fluid, to the caliper bleed port using the appropriate hose fitting. Ensure the hose is pressed firmly onto the nipple so it does not pop off under pressure. Having a secure, airtight fit at both ends is the only way to prevent introducing new air into the lines during the process.

Pushing the fluid

Open the bleed nipple at the caliper by a quarter turn. Gently push fluid from the caliper syringe up toward the lever. You will see old, discoloured fluid and tiny air bubbles emerge into the top syringe. Continue pushing until the fluid runs completely clear … this might take a couple of full cycles. Once the bubbles stop appearing, close the caliper nipple. Before tightening everything down, ensure you use a torque wrench to avoid stripping the soft aluminium bleed screws, which only require a tiny amount of force to seal.

Testing the lever pressure

With the system closed, remove the syringes and carefully reinstall the bleed port screws. Wipe down the lever and caliper with rubbing alcohol to remove any residual film. Pull the brake lever several times to test the feel. It should feel firm and immediate rather than spongy or soft. If the lever still pulls all the way to the handlebar, some air remains trapped in the line and you will need to repeat the bleed process from the beginning.