When a vacuum is pulled on an AC system that process empties the internal passages of air and moisture and any other gasses in the internal passages.
Pulling a vacuum is a good way to verify that the AC system has integrity and does not leak. If it holds a vacuum it should hold the pressurized refrigerant. But if it does not hold a vacuum, you won’t know where the leak is.
Pulling a vacuum is not the way to empty a charged or partially charged system. AC repair shops have equipment that bleeds the refrigerant out and contains it. That is the right thing to do.
To check for leaks in a charged AC system you need to add dye. You can buy AC refrigerant that has dye in it.
Typically you also need an ultra violet light. Inexpensive pen sized UV lights can be purchased.
