I’m looking for a term or phrase that is the inverse equivalent to the action of “topping-off”, like a gas tank, but meaning “to completely empty.” Not a steady state (empty, depleted), but more a phrase implying readiness.

Example: If going for a long drive and you want to tell your girls to go to the bathroom first (even if they don’t have to.)

Something that means exactly the inverse of “topping-off”.