While I had my engine out, I decided to upgrade the cooling system. I have always had trouble with the pressure bottle and getting the cap to seal. I had already ordered a new cap when I decided to make the change.
Even after repairing the main vacuum line in my HVAC controls I still had the problem that under prolonged acceleration, the vents would loose vacuum and switch to the Defrost setting. This problem is often caused when someone deletes the vacuum bottle behind the front bumper. However, my bottle is still in place behind the stock bumper.
I studied the vacuum diagram and saw there is a check valve where the source connects to the intake manifold. I tested mine by blowing through it. I could easily blow both ways so the check valve is bad.