How do I bleed the brake lines?

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Revision as of 17:19, 22 November 2006 by Payne (Talk | contribs) (Normal Method)

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Brakes are a very important system on your bike! If you're not completely comfortable working on this system, take it to a mechanic. Your brakes are vitally important, and if they're not working exactly right, it's very easy to get into an accident and possibly injure yourself or die. If you can't afford to take the bike to a mechanic, enlist someone from this board or a more mechanically-inclined friend to help out. This is not difficult work to do, but if you mess it up, you can kill yourself.

Keep in mind that brake fluid is corrosive to paint and some metals. If it gets on any part of the bike other than the inside of the braking system, clean it off as quickly as you can.

Brake fluid is also hygroscopic, which means that it readily absorbs water. So if it's exposed to water in any form (including moisture in the air), it'll absorb it. This lowers the boiling point of the fluid. As you brake, particularly in emergencies, your brakes generate a tremendous amount of heat, up to 300-400° F, which can cause "wet" brake fluid to boil. When it boils, pockets of vapor are generated, which act just like air in the lines, dramatically reducing braking performance right when you need it most. Brake fluid will usually list both wet and dry boiling points on the label.


Bleeding the brake lines is something which only needs to be done if the brakes feel spongy or weak. This can be caused by air in the lines, which compresses, absorbing energy but not transferring it to the brake pads. Weak brake feel can also be caused, to some extent, by warped brake rotors or contaminated pads. Always make sure you keep oil, soap, brake fluid or any other lubricant away from the brake pads and rotors. If the brake parts do get anything on them, clean it off with brake cleaner. Pads may not be recoverable if they get much oil on them. Replace the pads if you're uncertain.

Bleeding brakes will help purge air from the system. If you're having to bleed the brakes in between fluid changes, there's probably a leak in your system which needs to be addressed. Look for dark or oily spots along the system, and replace any parts you suspect.

Tools and Supplies

In order to bleed the brake lines, you'll need the following tools and supplies:

  • A #2 Phillips screwdriver (the most common size)
  • 10mm open-end wrench to fit the brake caliper bleed valves
  • Several rags
  • A bottle of clean, new DOT4 brake fluid (at least 8 oz/25ml)
  • Several feet of 1/4" vinyl tubing
  • An empty resealable bottle (milk bottle, soft drink bottle, yogurt container, etc.)

If you can afford it, a hand-operated vacuum pump makes this operation much easier and quicker. These cost around US$35, and can be found in most automotive parts stores; MityVac is a known and trusted brand. Harbor Freight sells the MityVac for US$35 and also sells another brand for US$20. As always, buy the best tools you can afford.

You may want to invest in a set of Speed Bleeders, which make this operation fairly simple while being cheaper than a vacuum pump.


There are three different ways to bleed the brakes, depending on which tools you have.

With all three procedures, remember that it is vitally important (as in, you could die if you don't) to keep the brake's hydraulic system clean and free of dirt, water, or any other impurities. Any foreign material in the braking system can result in the brakes failing to function either in normal operation or in extreme operation (like when you're braking hard and really need your brakes to work), or in them binding up. Any of these situations is obviously very bad; be as clean with the brakes as possible. It's ok if the outside of the braking system (like the hoses, reservoir, calipers, etc.) gets dirty, this is normal and safe. It's the inside you have to be concerned with.

Be aware that when you pull the brake lever, hydraulic fluid can shoot up out of the reservoir with a good bit of force. Remember that brake fluid will strip paint, so keep it cleaned up if any spills.

These procedures apply equally to the front or rear brake. They're mostly phrased as if working on the front brake, but you can replace "lever" with "pedal" in any of these instructions. The key differences are that the rear reservoir won't "spit" much or at all when you jam down on the pedal, and it's easier to get bubbles out, since the line is mostly horizontal.

Normal Method

Use this method if you have a stock bike, without Speed Bleeders, or access to a vacuum pump.

First, arrange your catch container. Attach the tubing to the brake caliper bleed valve -- it should have a rubber cap covering it. Remove that cap, and push your tubing onto the nipple. Route the tubing into the catch container, and pour about an inch of brake fluid into the container. This will allow backwards flow of brake fluid into the system without sucking in air.

Next, clean off the brake reservoir cap carefully, to get rid of any dirt. Take off the reservoir cap with the #2 Phillips screwdriver (or unscrew it if you're working on the rear brake). It's a good idea to put a rag under the reservoir in case of any spills, particularly with the front brake. Lay the cap and rubber diaphragm aside in a clean location, but keep them handy.

Empty the reservoir by either sopping it out with a rag or paper towel (be careful not to drip it on the paint!) or with your vacuum tool if you have one. Then refill the reservoir with fresh fluid. Be careful not to spill. Give the brake lever a very slow and gentle experimental squeeze. Note what happens in the reservoir. If nothing happens, try just a tiny bit harder and faster. Keep going until you see the brake fluid shooting up. For this reason, it's probably best to put the rubber diaphragm back on the reservoir, but you can do it open if you want (and are willing to clean up the mess).

Pump the brake lever sharply a number of times. You're trying to dislodge any air bubbles, so they're not "stuck" to the system, and will come out more easily. It may also be helpful to tap on junctions at the reservoir or on the caliper with a small wood or plastic mallet. A medium sized screwdriver handle (not the blade!) also makes a good light mallet for this. Make sure you're just tapping; the goal is to dislodge bubbles, not bend, dent or break anything.

Now grab your wrench, and prepare to open the bleed valve on the caliper. Before you do, put pressure on the brake lever with your left hand. Once there's pressure in the system, open the bleeder valve. You're going to close it a second later, just before the brake lever hits the bar. The goal is to only have the valve open when there's pressure actively in the system. As soon as the brake lever stops or begins its return stroke, it will suck in brake fluid (and probably air) from the bleed valve if it's open.

Repeat this operation a number of times.

  • Squeeze
  • Open-close
  • Release

Check often to make sure the brake fluid reservoir isn't getting empty -- if it gets empty, you'll suck air into the system, negating all the effort you just put in. You will probably see an air bubble or two come out of the valve. Keep going until the brake control feels firm and solid. There's a certain amount of flex inherent in the stock brake lines, so the system will never be truly rock-hard until you replace the stock lines with stainless steel braided lines.

The brake control should not sink at all, and should come to a stopping point and stay there, no matter how long you hold it.

Once the brake feels good, make sure the bleed valve is tight, but it doesn't need to be muscle-man tight. Too much torque and you could damage the brake caliper, possibly causing a leak and necessitating replacement. If you have a service manual, check the torque value listed. Top up the brake fluid reservoir if it's not already at the Max mark. Put the diaphragm and cap back on the reservoir, and screw it down. Clean up any spilled brake fluid, and you're done.

Speed Bleeder Method

This method is nearly identical to the Normal method. The only difference is that with Speed Bleeders, you don't need to work on opening and closing the bleed valve as you squeeze the brake lever. You just keep squeezing the brake lever until all the air is out. Keep a close eye on the reservoir, as it's very easy to accidentally empty it with Speed Bleeders.

When you open the bleed valves, they only need to be opened a half turn or so. Just enough to let the pressure escape. As you pump, you should see a bubble or two escape, although this will improve things even if you don't see any bubbles go by.

This discussion on Speed Bleeders may be useful.

Vacuum Pump Method

With a vacuum pump, you don't need to have a catch jar, although you'll want a disposal container. The pump should come with a fluid collection container. Set it up according to the instructions which came with the pump. Attach the longer (collection container input) hose to the brake caliper bleed valve.

Remove the reservoir cap and top up according to the Normal Method instructions above.

There are several ways to use a vacuum pump to do this. This article will concentrate on one good method.

Make sure the bleed valve is closed. Use the vacuum pump to pump up a good vacuum. Several full-stroke pumps with the handle should be enough. The vacuum gauge should register at least 1/5th deflection.

Now, with vacuum sitting on the outside of the bleed valve, squeeze the brake lever, and open the bleed valve with the wrench. Close it again before the brake lever hits the bar. This is the same as the Normal Method, with the important exception that the vacuum pump is also actively pulling out fluid and air. This will result in fluid leaving the system much faster than either of the other two methods, so you must be very conscious of the reservoir's fluid level.

Repeat this squeeze - open - close - release pattern a number of times, until the brake feels solid. You may see a lot of bubbles coming out; this is because the vacuum pump is actually pulling air through the bleed valve's threads. That's one reason not to pump up a vacuum and just open the bleed valve. (Another good reason is that with about three solid pumps on the handle, the pump will completely empty the Ninja 250's brake system in about 10-15 seconds.) You can avoid this problem to some extent by putting heavy grease on the threads of your bleed valve, but the seating face and inside of the valve must be completely clean of grease to avoid contaminating the brake fluid. Don't do this if you're not confident of what you're doing and why.

Again, once the brake control feels firm and doesn't sink under pressure, it's done. Top up the reservoir and put it all back together.

Cleaning up

Once you have the brake system feeling better, you're nearly done. Make sure everything is filled up to the full mark and tightened back down. You still have to do something with the old dead brake fluid.

Brake fluid is toxic, and disposal laws vary by state and city. It's not uncommon to find that just chucking it in the garbage is illegal. Check with your local laws regarding how to dispose of brake fluid, both new and used.

Your opened bottle of new brake fluid should now be considered contaminated (from water in the air). Only keep it if you want it around for emergencies -- it's better than not having brakes, but it'll go bad faster. Always use a new bottle of brake fluid when doing scheduled brake maintenance, if you can.

If you pumped very much new fluid through your system, you may have effectively changed the brake fluid. Please read the article on changing the brake fluid for more information. Note that the brake bleeding and brake fluid replacement procedures are nearly identical.