If the hydraulic disc brakes on your bike are feeling spongy at the lever, then it’s normally a sign that the system has taken in some air.
Unlike the incompressible fluid in your brake lines, the unwanted air bubbles are a compressible gas and will cause excessive movement at the lever and dramatically reduce braking performance.
Bleeding the hydraulic disc brakes on your bike may not necessarily need to be part of your routine maintenance, but it’s a fairly simple procedure and one that you shouldn’t be intimidated by.
Shimano hydraulic disc brakes are some of the most popular on the market and are also some of the nicest to work on. Follow our video above for a step-by-step guide on how to bleed your Shimano brakes. For your easy reference, a summarised guide follows below.
You will need
- Mineral oil
- Piston press or flat bladed screwdriver
- 2.5mm, 3mm and 4mm allen key, 7mm spanner
- Dedicated Shimano bleed kit
- Plastic bag, zip tie and rags to clean up
Mount the bike in a workstand and remove the wheel. Insert a flat-bladed screwdriver or a piston press between the disc brake pads and use it to push the pistons back into their bores. Remove the pad retaining pin, then pull the pads out of the caliper. Insert a Shimano bleed block.
Use a 4mm Allen key to loosen the brake lever bar clamp. Rotate the lever such that it is horizontal with the ground and tighten.
Use a 2.5mm Allen key to remove the bleed port screw on the top of the lever’s reservoir then use a pick to remove the small o-ring. This will sometimes come out with the cap.
Thread the Shimano bleed cup clockwise into the bleed port. Don’t overtighten this as it’s very easy to strip the threads on the cup.
Take the dust cover off the bleed nipple on the caliper.
Connect a bleed hose to the bleed syringe and fill with mineral oil. Invert the syringe it and squeeze out any air bubbles.
Put a 7mm ring spanner over the bleed nipple and attach the other end of the hose to the bleed nipple.
For non-series Shimano brakes, insert a 3mm Allen key into the bleed valve instead.
For all brakes, turn the spanner/Allen key anticlockwise a quarter turn. Use the syringe to force oil through the system. Stop before air enters the caliper then close the valve/nipple.
Remove the syringe from the end of the hose.
Put the hose, pointing down, into a bag. You can secure the bag with a zip tie, though make sure it doesn’t clamp the hose shut.
Open the bleed valve/nipple.
After half the oil in the bleed cup has drained out, close it. Pump the brake lever a few times, open the valve/nipple, squeeze the lever to the bar, close it, release the lever. Repeat four times.
Close the valve/nipple and remove the bleed hose. Tap all along the brake hose to unsettle any air bubbles. Flick and release the brake lever a few times. Repeat with the brake lever tilted forwards and then back by 30 degrees. Return the lever to horizontal.
Install the bleed cup’s plastic plug and remove the cup. Reinstall the bleed port screw and o-ring, taking care not to overtighten. Remove the bleed block, clean the brake caliper with a paper towel. Reinstall the pads and the wheel. Pump the brake lever to check it’s firm.
Jargon buster – What is Mineral oil?
Shimano brakes use mineral oil, as opposed to DOT fluid. It’s much less harmful to skin and can be stored longer without soaking up water from the atmosphere. DOT fluid can be more resistant to heat, though.