The expansion tank is transparent, which allows you to control the level of brake fluid in it at any time. During check-control, the brake fluid level drops below the Min mark. However, it is recommended to regularly glance at the expansion tank.
When the reservoir cap is closed, the brake fluid level should not be above the MAX mark and below the MIN mark (approximately 5 mm below or above the weld).
Fill only with DOT 3 or DOT 4 specification brake fluid.
Due to disc brake wear, the brake fluid level will drop slightly. This is normal.
If, however, there is a sharp drop in the fluid level over a short period of time, this is a sign of a fluid leak.
The leak location must be found immediately. As a rule, the leak occurs through worn cuffs of wheel brake cylinders. For safety, it is better to perform this work at a service station.
Checking the control lamp
Turn on the ignition, release the handbrake.
Press your finger on the contact in the cover.
Have the assistant check in the cabin to see if the indicator lamp lights up. If not, check the wiring according to the electrical diagram.
[Information taken from this resource «BMWMAN.RU»]