So you’ve got some fine-ass hydraulic disc brakes on your mountain bike. They’re great. No cable stretch! No messing around with tension! Excellent! They feel direct and firm and reliable. And then, at some point, they stop feeling like that. You can squeeze the lever right in after the pads have contacted the rotor. Or, as happened with mine, once the brakes heat up from use they lock on to the rotor and you have to sit at the side of the trail and wait for them to cool down before carrying on. There’s air in your tubes. Your brakes need bleeding. Simply, bleeding is the process of getting air bubbles out of a hydraulic system.
The process for bleeding brakes depends on the equipment you have and on your system. Essentially you add new fluid to one end of the system and remove it from the other, hopefully flushing out any air bubbles in the process. There are kits available which include a huge number of parts. After a little research I decided on the basic equipment and procedure below, but I suggest looking into it a bit yourself.
Stuff you’ll need:
Well this will vary considerably depending on the brake system you have, and on precisely what manner of bleeding you feel like doing. Here’s what I used.
Brake fluid. Use what the manufacturer of your brake system recommends. Shimano brakes use mineral oil, others use more traditional brake fluid. You need to use the right one as the wrong one will degrade the seals and cause corrosion in your system.
Syringes. Two of them. I had one that I bought in a ‘bleed kit’ and one that I bought from a farming wholesalers.
Tubing. It should fit tightly over the end of one syringe and over the bleed nipple on your brake calipers.
A spacer of some sort. I used a stack of plectrums. Other people have used credit cards, an appropriately sized hex key or a specific Shimano spacer block.
Torx drivers. The top of my master piston is secured by two T10 Torx screws, and the bleed cover by a T8 one.
Hex keys. You don’t have hex keys? How on earth are you maintaining your bike?
Step one: Prepare the bike. Set the brake lever so that the reservoir on the top is horizontal. You should also wind the reach of the lever out to its fullest extent, although I forgot to do this and it didn’t seem to affect things too much. At the other end, remove the brake caliper from the bike. This takes it nicely away from the rotor so you’re less likely to contaminate it with oil. Remove the brake pads (see the brake pads post for how). Push the pistons back into place and insert a spacer where the pads and rotor would normally be.
Step two: Open the top reservoir. Take out the two screws holding the top down and the bleed cover and lift off the cover, and the rubber diaphragm beneath.
Step three: Attach the tubing to the syringe and draw up a syringe full of oil. Uncover the bleed nipple on the brake caliper and attach the end of the tubing to it. Ensure it’s a good fit, it’s annoying to have it slip off. Once it’s secure, undo the screw to open the bleed nipple by about a quarter turn. Sometimes this means undoing the bleed nipple itself (and so you’ll need to make sure you have a spanner on the nipple before you attach the tube), but in the case of my brakes it’s a hex bolt at right angles to the nipple itself.
Step four: Clear the caliper. Push a few cc’s of oil into the caliper with the syringe, give it a couple of taps and then draw the plunger back a bit on the syringe. This should clear any air bubbles caught up in the caliper. It certainly did with mine.
Step five: Bleed bleed bleed. Now comes the bleeding itself. Slowly press the syringe attached to the caliper and inject oil into the system. At the same time, dip the other syringe into the reservoir on the handlebars and draw the plunger back, drawing old oil and air bubbles out of the system as you refresh it from the bottom. Carry on doing this until the colour makes it clear that it’s new oil going into the syringe and there aren’t any bubbles coming out.
Step six: Close the bleed screw on the caliper (gently, overtightening can damage the seals), and pump the brake lever a couple of times. It should feel firm even with the reservoir open. Then open the bleed screw again and inject enough to bring the level right up to the top of the reservoir.
Step seven: Reseal. Close the bleed screw again and detach your tubing. Put the diaphragm and the cover back on to the reservoir at the top – it should overflow a little bit. Screw it down nicely.
Step eight: Clean everything up. All the oil that leaks out or gets everywhere needs to be sorted out at this point. Particularly at the caliper end. Use degreaser or isopropyl alcohol to clean stuff down. The idea is that it shouldn’t have any chance to contaminate the brake pads once they’re back in. Speaking of which…
Step nine: Put the brake pads back in. See the post on changing brake pads. It’s basically the reverse of taking them out. Reattach the caliper to the frame, making sure the pads are properly set up round the rotor. Put the brake lever back to the position you want it in. Experiment with the brakes to make sure they still work.
A final note. I ended up bleeding my brakes twice in fairly quick succession. Anyone who read the previous version of this final note will recall that the first time it didn’t entirely fix the issue. The second time it did. Inevitably, the first time you attempt something like this you’ll make mistakes. I think I didn’t have the bike upright enough the first time, and the tube coming loose from the nipple didn’t help. It’s also possible that the seals on the brakes are a bit iffy, which means I should replace my brakes, but they seem to be working fine for the time being.
As always, if the first time seems to only be a partial success, try it again.