traditionally, automotive ac systems are tested for leaks by using a vacuum pump.
they evacuate all the air and wait a half hour to hour to see if the vacuum 'holds'.
this does 2 things:
1. shows whether or not the system leaks
2. evacuates the air (which is a very poor coolant) that needs to be removed before 'charging' for the ac system to work effeciently.

if you test the components for leaks separatly, you will miss the places where the leaks are most likely to occur - in the couplings