I have a 2000 Ford f250 Superduty and have been noticing a drips of brake fluid in the cab behind the brake pedal. With a closer look, it appears the "plunger rod" going into the rear of the master cylinder is wet. Can this be fixed with a rebuild or does it have to be outright replaced?

Before replacing the master cylinder, try changing the brake fluid. You may get lucky and flush any crud that's causing it to leak. I recommend changing brake fluid every two years, in order to purge the system of crud, moisture, air, and replenish fluid lubricity and corrosion protection.

Valvoline SynPower Brake Fluid meets Ford's HBH specification for heavy duty truck braking.