Hydraulic brakes work by applying pressure to a liquid in a tube. They work by means of a piston in the lever which shoots liquid into the caliper piston which causes the brakes to clamp. Because there is no stress caused, hydraulic brakes are considered more efficient and, in turn, more costly. If you ride hard on a regular basis, dirt and moisture can mix with the fluid, reducing the efficiency and so it is recommendable that you bleed your hydraulic disc brakes once a year. You’ll know you need to bleed them when there is no resistance when you grip your brake.
There are two types of fluid used for hydraulic disc brakes: DOT (Department of Transportation) approved automotive fluid and mineral fluid. THEY ARE NOT INTERCHANGEABLE. If you use the wrong fluid for your type of brakes, you will screw them over and your braking will be faulty. Now, you’ve got to remember that automotive fluid is toxic and can take the paint off your frame so be very careful with the handling. Make sure you wear gloves, work in a well-aired space and keep rags around the bleed area. Wipe off anywhere that has fluid, regardless of whether it is mineral or automotive, and wash the area with soapy water or alcohol after you’re done.
So to start, you’re going to want to set up your bike on a stand, remove your tires and take your brake off from the frame. You’ll want to remove the brake pads, the pin securing the pads and put in the shim or the caliper block in the caliper to hold the pistons apart. If you don’t have one, use a clean 10 mm hex wrench. Depending on whether you got the Shimano brake bleed kit or are doing it without will determine from where you start the bleed. With a Shimano kit: the bleed starts from the caliper up towards the lever (as shown in the video). Without: the bleed starts from the lever down towards the caliper.
Attach the bleed tube and the waste bottle (or syringe) and secure with a box wrench. You are going to want to open the reservoir above the lever so turn it so that it is level with the ground before opening. Remove the cap and the bladder and then give your wrench at most a ¼ turn to allow the fluid to flow smoothly.
If you are doing it with a Shimano bleed kit, depress the plunger to fill the reservoir with more fluid. If you are doing this without one, as soon as you open the bleed nipple, allow gravity to let the fluid drain from the reservoir to the waste bottle that is (hopefully) attached at the caliper end.
Start with only a little bit (5-10 cc) and then, suck it all back into the syringe. This is in order to make sure all the air bubbles that could have been trapped somewhere inside the mechanism is sucked back out. Once in the syringe, it’ll rise to the top near the plunger. Either way, you’ll want to keep at the process until the fluid is clean and you’ve got all the bubbles out. Try out the brake and see if there’s tension. Basically, just keep the draining process going until gripping the lever offers resistance.