There aren't many of you who haven't bled a Mustang's braking system while replacing or repairing your own brakes. Here's why you should bench-bleed a master cylinder prior to installation. Bench-bleeding off the car primes the bore with fresh brake fluid, eliminating air prior to installation to prevent the air from entering your Mustang's brake hydraulic system. It also prevents master cylinder piston jamming because you can observe the process hands-on in a vice.
You can bench-bleed your master cylinder using brake hydraulic lines or by purchasing a master cylinder bench bleeding kit from a local auto parts store.