Brakes air bleeding

Estimated time : depending on how much air there is in circuit...
Difficulty :
Required supply : brake fluid
Required tools : Syringe - pipe - jar
Check list : brake_bleeding.pdf

This page summarizes two different operations :

Brake fluid replacement, when too old. In such case, it doesn't support high temperatures due to presence of humidity, and it might vaporise. You should theorically replace it every 2 years (or more frequently if you use racing brake fluid like motul RBF 600 - every 3 month). You can see if liquid is used according to its color : light yellow when new, dark brown when out of age.
Air bleeding, if you have for example replaced brake hoses, or just feel there is some air in circuits (too much elastic brake command).

Be careful, if there is really a lot of air in circuit, it mght last very very (very ;-) ) long. Take new brake fluid, and open the filler cap of the master cylinder. Protect all parts around because brake fluid is very corrosive for paintings and plastics. You will have to install your 'bleeding tool' on the brake. Find the bleed valve.

Install a transparent hose on it (this transparent pipe may help you to see how much air is escaping the brake).

This pipe goes in a jar. Everyone may have its own technic to do clean job (I mean without brake fluid everywhere on the ground). Here is mine !

If you replace brake fluid, use the syringe to remove the old one in the master cylinder, and fill it with new one. If just want to bleed air, don't do anything.

Here begins the real bleeding action. The idea is to replace step by step the brake fluid which is in the circuit and to remove air in the same time. Here is the process, and it could be easier if you had a friend to help you.

Check that level in master cylinder is over the minimum (or you might re-introduce air in circuit).
Pull 2-3 times on the brake and let it under pressure.
Still pulling, open the bleed valve; some brake fluid (and air!) will escape.
Tighten bleed valve
Release brake carefully

And do it again ! On some bikes with complex brake circuits (like dual CBS), there is a special procedure. Check your service manual for more informations.

And there is another trap which can make you see air escape without ending. If the bleeding pipe is full, it can create a depression at the bleed valve. There is still a few air that might go beetween screw and brake body into your transparent hose. You think that this air was in circuit, and you keep bleeding....

Once brake is hard enough (and brake fluid which escapes is clean if you were replacing it), tighten bleed valve, adjust level in master cylinder, and check for the last time there is no fluid on the plastic and painted parts around.