I'm looking for a valve that I would describe as a "conditional valve." My application --- I have water going to 2 sources from a tee. I need source A to get preference over source B. So, source B will only get water when source A is not demanding any. Source A is going directly into a house, whereas source B is going into a holding tank that is not pressed for instant supply. It can be replenished slowly. I'm trying to avoid another holding tank at the source, and think this could solve my issue. The water source has a variable supply, which is my concern for both A & B using simultaneously. Worst case I just have to add a little buffer storage, but wanted to see if anything like that existed.