Do not worry too much about the length of time a vacuum will last in a test... .because many times the gaskets will do just fine in operational mode because they are supposed to be under pressure... they can be pulled away from their sealing surface enough to make it look like there is a leak under a vacuum.... 30 minutes sounds fine...
Other than that I may have to bow out of this because I don't have much experience with charging pressures.... and have NO experience with Duracool... All I have are the charts on R-12 to look at in the manuals....
Maybe you should hail REDFOX and ask his advise on this....

OR maybe change to R-12.... ?