8. Rear disc brake master cylinder: removal, examination and refitting

  1. To remove the master cylinder, attach a clear plastic tube to the caliper bleed nipple, open the bleed nipple by one full turn, and drain the fluid from the system by operating the brake pedal until no more fluid can be seen issuing from the nipple.
  2. While the right-hand footrest plate is still attached to the gearbox housing, slacken the locknut securing the master cylinder adjuster to the brake pedal, the brake pedal pivot nut and bolt and the two screws securing the master cylinder to the footrest plate.
  3. Withdraw the right-hand side panel and remove the single bolt or nut which secures the fluid reservoir to the battery carrier. Disconnect the fluid reservoir/master cylinder pipe at the hose clamp on the master cylinder top and withdraw the fluid reservoir.
  4. Remove its retaining Allen screws or bolts and withdraw the footrest plate, then unscrew the retaining nut and dismantle the brake pedal pivot. Unscrew the pedal from the adjuster, if required, otherwise peel off the rubber dust cover from the front of the master cylinder and withdraw the pedal and adjuster.
  5. Disconnect the brake hose at either end, then remove the master cylinder retaining screws and withdraw the cylinder assembly.
  6. With the dust cover removed, clean the master cylinder and look carefully for the fastener which secures the piston components. A circlip is usually fitted which can be removed with circlip pliers or prised out with a sharply-pointed instrument. depending on the type used. If the piston components cannot be extracted easily, take the assembly to an authorised BMW dealer and seek expert advice. Note however. particularly on very early K100 and K100 RS models, that if the master cylinder components are sticking or appear badly corroded, the complete assembly should be renewed. Unless the cylinder bore is in perfect condition, merely renewing the piston seals will not provide a safe long term repair; if there is the slightest doubt about any of the components of a brake system they should be renewed for safety's sake.
  7. Carefully examine the seals and piston, renewing them if there is the slightest doubt about their condition, although the seals should be renewed as a matter of course whenever they are disturbed, If the slightest trace of damage is found in the master cylinder bore, the master cylinder should be renewed as a complete assembly.
  8. Carefully clean each component and lubricate it with clean hydraulic fluid, then reassemble and refit the assembly following the reverse of the above instructions. To prevent water which leaks past the rubber dust cover from causing corrosion to form around the piston front end, pack the recess at the front of the master cylinder outside with silicone grease or a similar waterproofing agent which will not attack the rubber seals. Refit the dust cover. Pack the brake pedal pivot components with the specified grease (see Maintenance - Specification) on reassembly.
  9. Refill the system with clean brake fluid and bleed it, as described in Section 9 to remove all traces of air. Wash off all surplus hydraulic fluid and check that the brake operates correctly and efficiently before using the machine on the road.
  10. When refitting the brake pedal note that its height can be adjusted via the threaded adjuster and locknut, but check always that there is free play (which is measured most easily at the pedal tip) between the end of the adjuster and the piston. While BMW do not specify the required free play, there should be a very small amount discernible at all times or severe brake drag will result.

Fig 9.4