I think everyone has experienced this: they'd like to release liquid only after the ENTIRE system is full, and only then release the vapor/liquid into the surrounding environment or a tank.
At the moment, I haven't figured out how to fill an EMERGENCY tank as a last resort.

If you put a pipe sorter along the input line and priority into the tank you want to fill first, it won't divert any flow- assuming input and output lines are the same size- until after the priority tank is full.