How to replace the brake fluid
How to replace the brake fluid (or remove air from the brake system)
last update: 21/04/2001
Tools you need:
1. Screwdriver or 10 mm nut, for the side cover.
2. Philips screwdriver for the fluid tank.
3. 10 mm spanner for the draining bolt.
4. Transparent plastic hose (5 mm inner diameter), about 0.5 m long (two pcs if
there is a double rotor brake in the front wheel).
5. A suitable transparent draining bottle.
First, you must support the bike in a such a way that the front brake pump (on the handlebar) is in a firm upright position. Doing so, you avoid spilling the fluid out of this pump tank. A second measure is to apply a towel all over the front pump adjacent area. Brake fluid is extremely corrosive to all plastic parts, paint and, perhaps, tires too. The room must be clean, dirt is incompatible with any brake!
Then, you may start from the rear brake. Put
one end of the transparent hose onto the
corresponding draining outlet, insert the other end into the draining bottle. Remove the side plastic cover to locate the pump
and then carefully unscrew the fluid tank cap (2 screws). There is a white
plastic cover inside. Do not remove it, yet. Carefully loosen the nut that
controls the flow through the draining outlet. Do NOT remove it or loosen
excessively. If you loosen the nut only as much as it needs, fluid will flow
from the hose only. Otherwise, it will prefer to go out through the nut and
caliper body. In later case, fluid will exit fast and air will enter the
brake system (besides, the spilled fluid will make a mess all around the wheel).
Now, you can remove the plastic cover mentioned earlier. Then, gently,
press brake pedal until you see fluid flowing through the transparent hose. You
should see fluid level (in the pump tank) decreasing but be careful not to leave
the level drop to (or lower than) the bottom of the tank. This is vital.
You must NOT allow air to enter the brake circuit during the whole process. The
only way to achieve this is to assure that the there is enough quantity of fluid
over the bottom of tank.
When the level drops somewhat, fill the tank with new fluid. Repeat the above
pressing the pedal and filling the tank steps until all of the old fluid has
been drained
out and the new fluid has replaced it in the brake circuit. It's easier to
distinguish when the old fluid is completely removed, if the new fluid has a
different color.
Now tighten the draining control nut, re-install plastic inner tank cover
and outer cap, remove the transparent hose and proceed to the front brake. If
you have a dual rotor front brake, you must apply the above draining/filling
procedure twice, once for every caliper. Actually, it isn't all as hard as it
seems.
In case air has accidentally enter a brake circuit, then you
repeat the above draining procedure until you don't see air bubbles running in the
transparent hose. Again, the keys of success are:
a) the amount of loosening the draining control nut,
b) ensure that the fluid level in the pump tank is always over it's bottom,
c) press the brake pedal/lever gently in order to avoid sudden fluid level loss.
A nice tip for further helping to avoid air entering, posted by "Transalp" Eddie Hall:
"Before connecting the plastic hose to the brake fluid drain screw, leave the
master-cylinder (pump) cap on for a moment and remove the brake drain screw completely. Next, wrap a small amount of plumber's Teflon tape around just
the threads of the drain screw and replace it. Connect the clear plastic hose to it and place the free end of the hose in a suitable small container
with a few ml. of brake fluid in it. Having the tube end in the fluid will prevent air from possibly entering the brake system via the hose. The Teflon
tape acts as a seal to keep air from getting past the brake drain threads. You can leave the tape in place when you are through. (Ready for next time).
I got my tape at an auto parts store...near the brake supplies for only
about $1".