I would first make sure the small vacuum line going to the heater valve is connected to it and it is not broken (the line). Next I would crank it up and see if that line has vacuum at the connector that goes to the valve. Finally, I would suspect the valve is bad, but I would also check to the if the radiator is full to the cap after running it until it is warmed up and then cooled back down. Kinda doubt the air bubble on a buick if the radiator is full
