47-55.1 Need help find AC leak?

[PDW HOTRODS]

I am installing a new Vintage Air kit on my 1950 and I have a problem. I have evacuated the system and it will not hold a vacuum. I can put about 28" of vacuum in and close it and it leaks off in less than a minute. I have checked each fitting and O ring. No change. Then I replaced each O ring to be sure and still no change. I can't find the leak using vacuum. I know not to put air in it because of moisture, is there any other way to find the leak? Do I need to charge the system to find it? I thought you need to have vacuum in it to charge it? Thanks.