The pros assume that a leak means the system is corrupted... the cost of a new dryer/receiver is cheap insurance.
If the system is opened up at all a new receiver/dryer is called for. Also, it is the last thing to be put on the system before the vacuum is applied... and the cap should not be opened more than seconds before installation... it has moisture attracting stuff inside which is under oil.. so there is no way to take that out by vacuuming once it captures moisture....