Experienced bike owners and mechanics will tell you that brake bleeding is easy. It is, but there is plenty of potential for error. Reading this guide will not turn you into an expert overnight. We have made every attempt to be accurate and easy to read but we cannot impart the gifts of skill, experience and common sense. If after reading this page you feel inclined to carry out bleeding to the braking system of a bike we will not accept responsibility for what happens next. You are responsible for your own actions and this page has been placed online to only offer an introduction into bleeding.
Brake Doctor - What you will need...
Even though it is possible to bleed bike brake systems on your own it's advisable and much easier if two people do it. You will need clean, fresh brake fluid which has been allowed to settle over night - do not shake the bottle before starting as this will put air bubbles into the fluid, a length of plastic tubing which fits tightly onto the bleed nipple and a glass container so you can see the air being expelled from the system.
It's a good idea to cover any areas around the master cylinder and the bleed nipples to protect from accidental spillages. The area around the master cylinder and the bleed nipples should be as clean as you can get them as there is no point getting dirt into the system.
Firstly you need to remove the old lines so attach the plastic tubing to one of the bleed nipples and open slightly so you can pump most of the old fluid out before you take the old hoses off. It's not unknown for the bleed nipples to be seized in the calipers - mild steel nipples and alloy calipers will suffer electrolytic corrosion naturally and winter salt in the UK will only increase the effect. You may want to take the opportunity to replace the mild steel nipples with stainless steel ones.
Assuming that you have been able to undo the bleed nipples make sure the brake reservoir has plenty of fluid in it and then rest the cap back on top to stop fluid squirting out when you begin bleeding.
Brake Doctor - Priming (Filling) The System
If you have a twin disc system bleed one caliper at a time. Attach the tube to the bleed nipple and place the other end in the clean glass jar. Poor some clean brake fluid into the jar so the end of the tube is submerged so you don't pull air back in to the system.
Then open the bleed nipple, squeeze and release the brake lever slowly to give the master cylinder enough time to suck in fresh fluid from the reservoir.
Keep an eye on the master cylinder reservoir and make sure the fluid level does not fall below the minimum mark else you will start sucking air into the system. Fluid may be being pulled into the system from the jar and you may see the level drop - this is fine but again make sure the end of the tube is always immersed in fluid. It shouldn't take too many lever actions to fill the system. Tighten the bleed nipple when finished.
Brake Doctor - Bleeding The System
Open the bleed nipple slowly - you should only need half a turn and at the same time slowly and smoothly squeeze the brake lever in (or push the brake pedal down). Hold the lever in and you should see air bubbles or fluid being expelled into the jar. Old brake fluid looks foul and can be any colour from dirty white to brown or black. Movement of fluid and/or bubbles will continue for a second or two, close the nipple and then release the brake lever. Check the fluid level in the reservoir and top up if necessary. Repeat this operation until no more bubbles appear and the fluid coming out is clear.
If when you continue to apply pressure you get a slow movement or spongy feel to the lever it's a good sign that there is still air in the system. There are a number of possibilities not least that you didn't get all of the air out of the system so you should start bleeding again. Tighten all parts to the correct torque setting and then check the whole system to see that the lines are not trapped on full lock, no fluid leaks from anywhere etc.
Brake Doctor - Troubleshooting
Not all calipers have there bleed nipples at the highest point on their anatomy. This means that if there is a small pocket of air trapped above the nipple it will be hard to remove (air always goes to the highest point of the area it is in) and make the system spongy. You can get around this by taking the caliper off and making sure the nipple is at the highest point but remember to put a spacer in between the pads to stop the pistons popping out and making it easier to refit the caliper.
A similar problem occurs with some racing bikes which have steeply angled handle bars - the brake hose arches up above the master cylinder and a small pocket of air can get trapped here. Again you can rearrange the layout or you could inject brake fluid using a syringe very carefully and slowly in through the bleed nipple in the caliper bearing in mind that the fluid in the reservoir may overflow. Fitting a banjo bolt which incorporates a bleed nipple to the master cylinder and bleeding this first before the rest of the system is another way to fix this problem.
If you are unable to remove the sponginess no matter how carefully you bleed the system you may have a sealing problem which you will need to consult your local dealer about.
The master cylinder is fed from the reservoir by a tiny hole and this hole easily gets blocked which is why cleanliness is so important when bleeding brakes. If you cannot bleed your brakes yourself make sure you talk to your local dealer and get them to do it for you.
Don't be tempted to use any of the 'self bleed' gadgets unless you really, positively, nothing else for it, have to. These things allow the nipple to be kept open as they incorporate a non return valve to stop air re-entering the system. But the bleed nipple has a threaded end which screws into the caliper - air can be sucked into the caliper along this route if the nipple is loose in the threaded part of the caliper - it will only be a small amount but why do it in the first place as we are trying to remove air...
When you have successfully bled your brakes make sure both bleed nipples are done up tightly, all banjo bolts are done up tightly and top up the reservoir. Do not overfill the reservoir as this can cause hydraulic locking of the system preventing the pistons in the caliper from fully retracting - this causes binding of the brake.
Re check the system visually before test riding - and we mean test riding. Just go forward a few feet slowly and apply the brakes then bring the bike back into the workshop to check that their is no fluid leaking from the system, everything is done up correctly and the brakes have a good solid feel to them. Do not ride your bike until you are certain you have bled the brakes correctly - If in doubt get your local dealer to bleed the system for you.
Brake Doctor - Troubleshooting And Trade Secrets!!!
Bleeding brakes like many things is a skill and with some modern master cylinders being mass produced sometimes rough areas within them can catch and air bubble which you simply can't remove with normal bleeding procedures. In these cases you can use our handy 'trade secrets' which may well help.
1 Tying the lever back overnight
One way to rid any last remaining air from the system is to tie back the lever over night. Firstly remove the master cylinder reservoir cover BUT then balance it back on top of where it should be fitted so that moisture in the air does not contaminate the system overnight. Then pull the lever in as hard as possible and cable tie it in the ON position (brake lever pulled right back to the bars). Leave this overnight and in the morning release the lever and your brakes should feel fantastic - basically the back pressure caused by tying the lever back overnight will have forced any last remaining air out of the system out to the atmosphere and leave you with sharp brakes the next day. Replace the reservoir cap correctly and you are ready to ride.
2 Priming the system the easy way
You will have read above the normal way of priming the system but another way (used by many professionals) is to connect the hoses to the master cylinder as per normal but instead of fitting them to the calipers you simply put the caliper end or ends of the brake lines INTO the reservoir at the top so that the banjo holes or fitting hole is beneath the surface of the brake fluid. Make sure you have towels etc covering all areas around the master cylinder in case of spillage etc. SLOWLY pump the system in and out (not fast as brake fluid will squirt out over paintwork etc) as this 'circuit' will completely fill the hoses with fluid making bleeding much easier afterwards. On the final 'squeeze' of the lever leave it on (fully pulled in) and tie it back. Remove the hoses from the reservoir and wipe off all excess fluid. Then connect them to the calipers at the correct torque and then release the lever. Bleed as normal but with the added benefit of the lines and master cylinder being air free to start with!!!
3 Reverse Bleeding
Although we aren't fans of this type of bleeding I do include it here as many bike shops use this themselves - the only thing I would say is if you push the fluid in too quickly you can pop brake caliper seals etc so slowly and low pressure are the key words here!!!
For this you will need a large syringe filled with new fresh brake fluid and a tube which fits the bleed nipple and the syringe. Connect the syringe tube to the bleed nipple and open the bleed nipple - SLOWLY push the brake fluid in through the caliper up to the master cylinder (which is bleeding in reverse) Do the caliper furthest from the master cylinder first and then the other one. Repeat if necessary and keep checking the reservoir level so you do not overfill the system. This way ALL AIR IS PUSHED UP AND OUT OF THE SYSTEM meaning bleeding is much easier than trying to push air DOWNWARDS and out of the system in conventional bleeding BUT please do not exert massive pressure at the caliper as this can pop seals etc.