I wouldn't use air pressure except as a last resort. Your cooling system has probably never been properly flushed since new. The first thing to clog up is usually the heater core, because it gets no flow unless the heater valve is open and sits idle at least 1/2 the year corroding up. Do a chemical flush via the directions through the whole system first, see what happens, then do your specific heater core stuff if needed. Just my recommendation.....