The brake fluid reservoir is usually plastic and clear or semi-translucent. This allows you to visualize the brake fluid levels without removing the cap. The reservoir is usually in the back of ...
If the brake fluid in the reservoir is dirty or brownish or if the brakes start feeling ineffective, have the brake system checked and have the fluid replaced whenever possible.
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.