Dying Dart after being warmed up.

I'm thinking vacuum leak, or low vacuum because I also recently add a power brake booster and it feels like it takes awhile for the booster to "charge up". Like at cruising speeds that first tap on the brakes stops like a dream, but at idle or if a pump it a bunch rapidly it looses the "powerness" of it.

Random, stupid question...whats the best way to cap off unused vacuum lines? Also anyone know the best spot I should be tapping into for my brake boosters vacuum line?
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