Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at firstname.lastname@example.org. The latest Art’s Cyclery/VeloNews Ask a Mechanic Video is shown first followed by an answer to a question on how to bleed Shimano brakes without a bleed kit.
Question: I was hoping you could do segment on bleeding Shimano disc brakes without their bleed kit or the simplest way possible. From: Mark
Answer: Bleeding disc brakes without a bleed kit is always less than optimal, but sometimes you get in a situation where you need a brake bleed and have a ride in an hour and don’t have a kit, or you are out on a road trip and don’t have the kit with you. Fortunately, you can actually get a pretty decent bleed on Shimano disc brakes without using the bleed kit.
Start by removing the caliper from the bike and letting it dangle as low as possible to help the air bubbles make it up to the lever. Rotate the lever so that the bleed port is facing up. Remove the bleed port screw, or master cylinder cover on older brakes, and top off the mineral oil. Remove the brake pads and insert a bleed block to hold the pistons in place. If you don’t have a bleed block then the caliper needs a rotor to clamp down on. A stack of business cards will work too. Now, start pumping the brake lever. Intersperse the lever pumps with what I call a lever flick. Pull the lever all the way back and then let your finger slip off the lever so that is snaps back to fully extended. As you pump and flick the lever, tapping the brake line and caliper will help to dislodge air bubbles and move them up to the top of the lever. You may need to top off the mineral oil again when you are done.
A second option is to bleed the brakes like a car if you have a stubborn bubble stuck in the caliper. Having a friend help you with this one will make it far easier and cleaner. Start off the same way with the caliper off the bike, brake pads removed, and bleed block in place. Open the lever’s bleed port and top off the fluid. Now pull the brake lever; while you have the lever depressed, open the caliper bleed port and let the lever go all the way to the handlebar. Mineral oil will squirt out the bleed port so have some rags ready. With the lever still fully depressed, close the bleed port on the caliper, then release the lever. You can repeat this process one more time before you will need to top off the mineral oil again. Continue the process until the brakes firm up. Be sure to wipe off all the excess hydraulic fluid before reinserting the brake pads.
This same process can work for other disc brake systems, but I have found that it works best on Shimano brakes. Although I already said this before, doing the job right is always best. All of Art’s hydraulic disc brake bleed kits are available by clicking on the highlighted link.
Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.