Bleeding brakes is one of those tasks that is really simple, but can be a real pain if you don't do it correctly. There are ways to be a little more efficient by using a vacuum pump, but since most of us don't have an $80 vacuum lying around, we're going to talk about how to bleed brakes manually. Before we get started, there are a few things to note. First, there are only two reasons you should bleed your brakes. If you have a loose brake line (which is why we are bleeding ours) or if you just replaced a component that directly affects your brake line. If you just have a spongy feeling brake, you more than likely have air in the system and will need to address that issue before attempting to bleed your brakes. Second, you will want someone nearby who can assist. (Don't worry, they only need to squeeze the brakes.) The very first thing you need to do is fill the brake fluid reservoir.
Make sure you add as much brake fluid as you can since it will be going down during the bleeding process. While DOT 4 is a very common type of brake fluid, there are always exceptions. This is another instance where you will want to check your owner's manual to make sure you are using the recommended brake fluid. After filling the reservoir, I went to one of the front calipers and loosened the bleeder screw then snugged it back.
I do this on both sides to make sure that they are not so tight that I have issues getting them loose. It also helps to verify that I have the right size wrench. [alert-announce] A note on the wrench: When bleeding the brakes you will need to use a boxed end wrench or a flare nut wrench. Using an open end wrench makes it too easy to round the hex part. Using a ratchet makes it difficult to see if there is air coming out of the system. [/alert-announce]
After I know that I can break the bleeder screw loose, I pump the brake handle. Since our brakes were completely dry, there was very little pressure on the brake handle. In the photo below, you can see it was pretty easy to squeeze the brake handle in all the way with very little resistance.
Now here's where your helper will come in handy. I had Beth hold the brake handle down while I broke the bleeder screw loose. Once you've opened and closed the bleeder, your helper can let go of the brake. This will push air to the caliper and hold it there. When you open the screw, the air is released and brake fluid starts getting sucked down the line. You will need to repeat this several times. Usually, I will go through this process three or four times on one side and then three or four times on the other side to get most of the air out of the line so the master cylinder will start to get pressure against it.
Eventually, you will start to see brake fluid coming out of the out of the bleeder screw. While you are working through this process, make sure you are keeping an eye on the brake fluid level. You will know that all of the air is removed and you are done bleeding the brakes when there are no air bubbles coming from the bleeder and when the squeeze on your brake handle has tightened.
Before you finish up, make sure that you check the brake fluid one last time. On the first test drive you'll want to take it easy and be sure that your brakes aren't loosing any fluid. The brake handle should remain stiff and braking should feel smooth. It is possible to just need to bleed the brake on one side, but this is only if you only have removed the line or a component below the split in the line that comes from the master cylinder. Otherwise, you will need to bleed both sides. I usually bleed both sides regardless. It only takes a few extra minutes and gives me the piece of mind that they were both done correctly. Have you ever had brakes that gave out when riding? What was your worst experience with bad brakes? Do you have a better way of bleeding the brakes?