Level Check

WARNING: This page is about a different car, the 2006 Nissan Titan. However, it is still accessible from the selected car via links, so may be relevant.

Make sure the fluid level in reservoir tank is within the standard (between MAX and MIN lines).
Visually check around reservoir tank for fluid leaks.
If fluid level is excessively low, check brake system for leaks.
If brake warning lamp remains illuminated after parking brake pedal is released, check brake system for fluid leakage.

Fig 1: Checking Brake Fluid Level
Courtesy of NISSAN MOTOR CO., U.S.A.