Use brake fluid test strips or a pen tester to measure ... will completely flush all the old fluid. If you continue, fill the ...

The brake fluid reservoir is usually plastic and clear or ... Wash your hands if exposed, and wipe it off of painted surfaces immediately. Fill to the maximum line. Secure the cap tightly.

but like brake calipers, sometimes it can spring leaks or otherwise fail, and it must be rebuilt or replaced. The master cylinder is topped by the brake-fluid reservoir, which must be kept full.