as you have experience with the adjustable PVC valve, what kind of difference did it make ?
i know each combination is/will be different, i'm just curious as to how that affected your combination as it is now.
a vacuum pump may be in your future. you might try an electric GM pump first, as they are relatively [?] cheap compared to other brands, and you may be able to grab a used one from a yard near you if possible, to see what a pump will do with your combination.
thank you for keeping us informed of your progress.