BMW R850 Maintenance Manual page 45

Attach the drain tube and jar to the rear caliper nipple and repeat the four steps with the
rear brakes. Gently and carefully actuate the rear brake while releasing the nipple on
the rear brake. Observe the rear reservoir closely and replenish as needed.
Discontinue pumping when the fluid is clear; close the nipple as fluid is gradually flowing
out.
After the fluid has been completely replaced and all nipples are tight, check that the
front and rear reservoirs are filled and secure. Slowly pump the brake lever, then pump
the foot pedal until there is a solid feel. Continue pumping the brakes for a few minutes,
then recheck the reservoir level and refill if necessary.
WARNING: You must pump the brakes and be certain the brakes are responding with a
solid feel BEFORE you ride the bike!!!
TROUBLE SHOOTING
AIR IN THE SYSTEM --- The brake system works with brake fluid. You push on the
brake lever and this force is transmitted to the brake piston at the discs and causes the
pads to grip the discs. The more you pull on the brake lever, the more pressure you put
on the discs and the faster you stop. There is fluid in the brake lines that transmit this
force; you cannot compress this liquid fluid. But if there is air in the brake lines, even a
little, this will cause problems because air compresses. If you pull on a brake lever and
there is air in the brake line, the air will compress and less force will be transmitted to the
brake pads.
If you feel air in the brake lines (spongy brakes), you MUST bleed the brakes until all of
the air is expelled!!! If air is allowed to remain in the line, it will compress when the
brakes are actuated and will not give you firm and steady pressure on the pistons and
pads. This will result in uneven and dangerous braking. Soft or spongy brakes feel as
though they are not working because they are not working. The best procedure is to
work so that no air enters the system. If it does get into the system, you must expel all
air from the system.
AIR IN REAR BRAKE SYSTEM
If you get air in the brake system, especially the rear brake system, you will want to start
the corrective process by bleeding from the ABS nipple under the tank. Do this because
air bubbles, which rise, may be trapped in the higher ABS area. Then continue with
either Method One or Method Two. See: TWO OPTIONS.
UNRESPONSIVE BRAKES
If you still have an unresponsive brake system after bleeding the brakes and after
following the procedures recommended here, you may have water in the system or an
ABS problem. Do not ride the bike! Seek professional help.
TWO OPTIONS
The ABS system has a bleeding nipple under the gas tank. You do NOT have to bleed
from this nipple. Old brake fluid will be totally flushed when bleeding from both front and
rear caliper nipples. Bleeding from under the tank will neither hurt nor help your brake
system. The nipple at the ABS unit under the tank may be bleed before starting with the
front nipples at the calipers. Some shops bleed at the ABS under the tank, some do not.
Under tank bleeding is recommended by the BMW manual. However, you should bleed
from under the tank if your bike experiences problems outlined in the paragraphs: " AIR
IN REAR BRAKE SYSTEM" or "UNRESPONSIVE BRAKES'
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