A/C system?

What you saw there is more the DIY top up on a low charge more than the full process. To find the leak you can charge the system with any dry gas, I use CO2 normally (it's cheaper) then go over every fitting and core with soapy water and look for the bubbles. I know of folks who use shop air as well but it must be dry shop air with no oil in it, I've used one of the small pancake compressors with two drier filters on the output. Then you do the same, if you want something that bubbles and sticks a bit better the bubble liquid, they sell in the dollar stores works good. Now once you find and repair the leak you would pull a vacuum. Because you had the system open leave it under vacuum until it holds. That will boil the moisture out of the drier and any that was introduced with the air. If you watch it you will see the pump pull it down, then it will rise a bit as the moisture comes out and then start bulling vacuum again. Because you had it open you could pull it down, let it set, then pull the vacuum again. That way you don't replace the drier again. When you charge it add some UV dye to the system, will make it easier to spot future problems.
If you have access to a machine, it's even easier as most have a way to cycle them if moisture is suspected.