Can you not just use the vacuum pump on the car? If it is capable of pulling better atleast 25 inHg, it will work fine for purging the A/C system.

There is a chart somewhere on the internet which shows the evaporation point of water at various ambient temperatures and pressures. It seems like 21 inHg or so was needed to evaporate water near room temp.

Just a thought... it might be a convenient way to purge the system if the pump is up to the task.

-Joe