The brake fluid reservoir sits on top of the master cylinder (silver metallic object below) and this keeps the brake system filled with brake fluid. This reservoir screw cap incorporates a dipstick on the underside of the cap. The large black circular object on the right is the vacuum booster that provides power braking for the driver via a vacuum operated diaphragm that helps push the plunger into the master cylinder to hydraulically operate brakes on all four wheels. If you suspect a brake fluid leak, the interface between the master cylinder and vacuum booster is one place to check.