| Introduction
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 brake system of your bike we will not accept responsibility of what happens next. You are responsible for your own actions and this information is offered as an introduction to bleeding.
Lets start at the very beginning.
STEP 1 - Before You Start!
You will need... Even though it is possible to bleed the brake system 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 overnight - 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 air being expelled from the system.
It’s a good idea to cover areas around the master cylinder and the bleed nipples to protect from accidental spillage. The area around the master cylinder and the bleed nipples must be as clean as possible to avoid 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 can take the hose off. It’s not unknown for the bleed nipples to be seized in calipers- mild steel nipples and alloy calipers will suffer electrolytic corrosion naturally and winter salt on the roads 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.
STEP 2 - Priming (Filling) The System
If you have a twin disk 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 into 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 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 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.
STEP 3 - 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 brown or black. Movement of fluid and/or bubbles will continue for a second or two, close the nipples and then release the brake lever.
Check the fluid in the reservoir and top up if necessary. Repeat this operation until no more bubbles appear and the fluid coming out is clear. Remember to keep the master cylinder topped up.
If you have a twin system repeat this process with the other caliper (It’s best to do the furthest away from the master cylinder first) if everything has gone okay you should now have a brake system with a good solid feel to it, the lever will travel a short distance and then a solid resistance will stop it moving any further.
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 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.
STEP 4 - Troubleshooting
Not all the calipers have there bleed nipples at the highest point on their anatomy. That means if there is a small pocket of air trapped above the nipple it will be hard to move (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 cleaning 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 are really, positively, nothing else for it, have to. These gadgets allow the nipple to be kept open as they incorporate a non return value 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 the 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 the air…
When you have successfully bled your brakes make sure both bleed nipples are done up tightly, All banjo bolts are done up tightly to the correct torque settings and top up the master cylinder reservoir with the brake fluid to the required level. Most oem reservoirs have a upper and lower limit shown on the reservoir itself. 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 brakes.
STEP 5 - Final Check
Re check the system visually before test riding- and we mean test riding. Just go for a few feet slowly and apply the brakes then bring the bike back into the workshop to check that there 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 that you have bled the brakes correctly- if in doubt get your local dealer to bleed the system for you.
Check that all end fittings are securely attached to each line. Check line (s) for clearance and that the kit has now been installed without any kinks or twists in the system. Check that full suspension travel and steering lock are unaffected and that the hoses are not stretched or trapped in any way. |
