Brake Fluid Reservoir and Vacuum Booster
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.