i just flushed my radiator and everything was working great. the other day the over flow tank started bubbling out. i replaced all the hoses and thermostat. i was wondering if air can be getting in the system through the heater box. i think it has a leak, because there was some antifreeze on the floorboard of the passenger side. how hard is it to replace it, and could this possibly fix my problem? thanks for the help in advance