Pull the vacuum, isolate the system with the core tools, remove the vacuum hoses, purge the refrigerant hoses, install them on the core tools and weigh the gas into the system as usual. After you are finished put the core back in purging the core port with the system gas (a fraction of an ounce) and then start the system. I never pull through a manifold. There are too many leaks. I always use vacuum rated core tools and hoses, even on a window unit.