This is the brake master cylinder found on all 350Z models equipped with Base brakes.
The brake master cylinder is responsible for converting the mechanical force from your feet to hydraulic force to compress the brake calipers. When the brake pedal is depressed, brake fluid is forced through the brake lines and compresses the pistons and brake pads to stop the car. Over time, the seals in the brake master cylinder break down and can allow the fluid to pass the seals, resulting in a loss of brake pressure.
For drivers looking to increase driver feel and reduce firewall flex, Z1 offers a master cylinder brace that can be used with all OEM and aftermaket brake master cylinders found HERE.