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For the last few years I've had a problem with my airconditioning. This problem also happens with the setting of vent and so on.
When I am traveling with the vent or a/c on and I hit the gas/speed up, the air flow switches from the normal vents to the vents used to defrost the windshield. At first I thought this problem was only during use of the airconditioner, but I have found that when using the vent setting or others it happens when excelerating.
I'm dreading the summer time with a 50% airconditioner
Sounds like you have lost vacuum to the canister, which I believe is in the passengerside fenderwell. It keeps vacuum on the system under acceleration when manifold vacuum drops. Check the plastic vacuum lines for cracks.