Is master cylinder leaking where it bolts to the power brake booster? If so you have a choice. 1. Buy a new master cylinder or 2 Get a piston kit from Nissan and rebuild the old cylinder. If you decide to rebuild you will need a 3 stone 1" cylinder hone to clean up the piston sleeve. In additon after installing the new pistions and seals you need to fill the fluid feed openings with brake fluid and move the new pistons and seals until they move smoothly. Then re-install the cylinder and bleed the system. MaxDoc