I've been told that pulling a vacuum for several hours (pump on for that long) makes sure to get out any residual moisture in the system. I do that, but can't verify that it's imperative. Leaving a system with a vacuum seems better that releasing it if it's just going to sit for a few days. That would pull moisture back into the system.

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1985 300D-189k The 'UD', Ivory and Pinkamino
1979 300D-211k Dark Gray, Parchment
A 1980 Harley-~166k and
A 1994 Ford diesel pickup-349k and
A 1990 gasser Volvo wagon-145k