Weak Defrost

[kenm78]

My heater in my 82 works good will roast you out. But the defrost is lousy. Hot air comes out of the dash vents great, and when switched to the floor it comes out nice and strong. When you switch it to defrost its nowhere near the flow that comes from either the dash vent or floor. I checked the vacuum valve near the throttle pedal right at the floor heater exit and it is closing all the way when you select defrost just not much air comes out of the defrost vents. I'm tired of slow defrosting. Any help or suggestions would be greatly appreciated. I'm tired of chasing my tail here.