Whenever I have the air turned off on my 84 Blazer, heat comes out of the heater vents. It blows pretty hard. Turn the air on or just the "vent" setting on and it stops blowing hot air out the heater. Is something not closing all the way allowing heat to get by? Thanks 1984 K5 Blazer w/ 33" BFG All Terrains, 305, auto, no lift. 4" lift and 350 coming soon.