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.

You can use a balancer to do the same thing. Set priority on where you want it to go and have the excess go elsewhere

... Yeah, that's what I meant. It's been a few days since I last played, it's easy to forget the names. Pipe sorter doesn't exist in vanilla, balancer very much does and is what I was describing. I use it all the time to ensure that my diesel-fed petroleum refinery (via the diesel-gold-crude trade loop) prioritizes feeding itself over sending fuel to the train for the rest of the factory, so that it can't possibly be starved.