It would be nice to have a storage that has keep full set to prevent connected belts from pulling items below that level so that it can be used as a buffer.
E.g when making construction parts belt them into storage, then belt that storage into asembly machines for constructon parts 2, with this setting you could keep a set amount of parts 1 so that trucks could use them without parts 2 using them all
Attached is a quick example of how it could look. The same should apply to fluid and loose storage
