When I accelerate, the airflow goes from the dash vents to the defroster. When I let off the gas, it returns to the dash vents.
The AC has a vacuum reservoir that is designed to hold a vacuum to the AC blend doors during acceleration, or low engine vacuum conditions. If this system is leaking, the system cannot hold vacuum to the AC, and will default to the defroster vent when vacuum levels drop. I recommend that you have a mechanic, like one from YourMechanic, check for broken vacuum lines, or a leaking reservoir, through an inspection of your entire AC system.
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