Before starting work on any component of a hydraulic brake system, note the following:
Caliper overhaul should be preceded by removing the pads. as described in Routine Maintenance.
Check carefully with a BMW dealer exactly what replacement parts are available to recondition the components; there is no
point in attempting to dismantle an assembly if lack of pistons and seals means that it cannot be repaired. On the front brakes, both calipers
should be always overhauled together to preserve brake performance.
Remember that brake fluid is an excellent paint stripper and will also attack plastic components. If any is spilt, wash it
off as soon as possible with fresh water. Use only new fluid from a freshly-opened' sealed container as it is hygroscopic, which means that it
absorbs water from the air. This eventually lowers the fluid's boiling point to an unsafe degree; fluid should never be re-used.
The brake discs can be checked for warpage by clamping a dial gauge to the fork leg or final drive housing; if runout
exceeds the maximum limit specified, the disc(s) should be renewed. If they are worn at any point to less than the minimum specified thickness,
or if they become scored for any reason, braking efficiency will be impaired. The discs should then be renewed; they are held on by bolts,
which pass through the front wheel hub, secured by nuts. On refitting, ensure that these are tightened to the specified torque wrench setting.
The rear brake disc is fastened to the final drive crownwheel flange by two countersunk screws which are treated with thread locking compound.
They were found to be tight enough to distort noticeably under the pressure required to slacken them; owners would be wise to have new replacements
on hand in case similar problems are encountered. Be careful to degrease and clean the mating surfaces and before refitting the disc, also the
tapped holes in the crownwheel flange. Tighten the retaining screws to the specified torque wrench setting.
Examine the flexible hoses for cracks or scuffing and the metal pipes for cracks or corrosion. At the first sign of damage,
they must be renewed, First drain the system. Unscrew the unions at each end of the hose and remove the hose, releasing it from any clamps or guides.
After fitting a new hose, tighten it securely, re-fill the hydraulic system with new, clean fluid, and bleed the brake as described in
Section 9 of this Chapter - Bleeding the hydraulic brake system.
Check all unions for tightness. The hoses and brake pipes must not rub on an adjacent part.
If fluid leaks around the brake pads, the caliper seals are faulty. The handlebar lever will feel 'spongy'. Complete failure
of the brakes, although there is pressure at the handlebar lever, may indicate a seized piston. In either case, the unit must be removed for servicing.