replaced the master cylinder on 2003 F150 now the brake warning light is on?

 

Did you bleed the brakes all the way? What did you use to bleed them (2 people, 1 man bleeding system)?

 

bench bleed the master haven't bleed brakes yet.

 

That could be why then. The ABS system is a PITA. IF it's not bled right it can throw a code for the ABS system. Basically any air in the system will make it read the pressures incorrectly. Fully bleed your brakes then reset the code and see if the light still appears.