So far as i understand it priorities are used for two things:
Deciding where to store something and
deciding, which recipe to use.

What i want is to have the contents of one Storage bus to be used up completely before ones with lower priority are even scratched.

For example i have two sources A and B for water and one consumer. both storage buses are set to extract only. Currently setting A to 1 prio and B to 0 seems to be doing nothing at all, while it should lead to B being unused as long as A is not empty.

Extract is reversed to automatically cleanup an actively used system over time.