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.