The clutch and brake master cylinders share a fluid reservoir. Note that a leaking clutch slave or master cylinder will lower the reservoir level just as easily as a leaking brake hydraulic system.
BMW recommends that the brake fluid be changed and the system bled every 2 years. It may be wise to increase that interval to once a year for increased protection from hydraulic system damage. If the vehicle is used for sporting purposes, it is wise to change the fluid after each event.
Use only high quality, name brand DOT 4 brake fluid. Use tightly sealed small containers to refill the reservoir. Large containers offer a false economy. A large container that has been opened will absorb moisture and ruin the fluid. Only use large containers when performing major work on the brake or clutch hydraulic systems and the entire contents will be used at once.
Tightly cap any container to prevent moisture from being absorbed into the brake fluid.
See Figure 1
Wipe the reservoir cap and surrounding area clean. Make sure the level is up to the full marker (the reservoir is translucent). If necessary, add DOT 4 specification fluid that is brand new (do not attempt to reuse fluid). Be careful not to drop any dirt into the fluid, and avoid spilling fluid on the paint work, or wipe it up immediately if it spills.