1975 Ghia with factory air. If heater/AC control is set to Vent air flows OK when the car is idling or running at highway speed. However, when I accelerate the air flow moves to the defroster. After I let off the gas or the car gets up to a steady speed then the air flow goes back to the dash vents. Other control positions have the same type of failure. I figure it is a vacuum pressure problem (since vacuum drops when accelerating) but do not know what is causing it. I just rebulit the car and all the hoses are new. Checked for leaks and found none. Whats keeps vacuum pressure steady on the HVAC controls when the engine vacuum changes??