I would first try to bleed all the brakes (in the proper order). Then if you still have problems, look into replacing the master cylinder. Air in the system can lead to a soft brake pedal. However, if you did get air in the system, you have to wonder how it got there. If your fluid level got too low due to a leak, somewhere, it could have sucked some air so it could still be that you need a master cylinder if it was the cause of the leak.