You’re no doubt familiar with using a vacuum to test an A/C system’s ability to retain its refrigerant without leaking. Before recharging an air conditioning system, you put it under deep vacuum. If it holds for a reasonable length of time after the vacuum pump has been turned off, you know it’s safe to add refrigerant and finish the job. If the system doesn’t hold a vacuum, there apparently is a leak. The deep-drawn vacuum also removes any air that may have accumulated in the system.