If something is partially seized it could, seals leaking not having maximum effect of the vacuum.
If the seal in the master cylinder that blanks off the reservoir is not sealing correctly then the brake pedal continues downward until either it does seal or the pedal get to the floor and the car does not stop. If the pedal acts as normal then it would seem that the master cylunder is acting as it should.
If the car pulls up, eventually, square then that to me would suggest that the calipers are working correctly otherwise if one was having an off day then it would pull.
The servo is the item that adds the power so either a lack of vacuum or a problem inside. Wiki explains the operation.http://en.wikipedia.org/wiki/Vacuum_servo