Sorry to revive an old thread, but I came here in hopes of a simple answer. In the year 2022 (1.12.2 modpack), here's a simple way to purge excess liquids. Attach a clay pipe to your storage tank, then continue the pipe into a void pipe. If you want to consume the liquid, continuously pump it out of the tank with a redstone engine. The void pipe will only get liquid if the storage tank is full.