Remember to run the engine, drive it around, whatever and get it up to full running temp under the hood. Then shut it down and hook up the vacuum pump. The hotter the engine and engine compartment is when you start evacuating the system the more moisture you'll pull out with the pump. The pump isn't just to evacuate the system of air as best it can but to also pull all moisture out of the system as well. Higher temp + vacuum = lower boiling point of water in the system under the vacuum the pump can create. You get far more moisture out of the system if you place it under vacuum when everything is hot.