Could you make a one way valve that only lets fluid flow in one direction if the input fluid is lower than the output fluid so you can prevent the pipes from trying to fill up pipes that are providing a fluid to a main pipeline? It would be really helpful.

Hello! Not sure I understand why you would do that. So if the input is lower then the output, then it will push the fluid into the output, making ithe input even lower. And probably you'll never be able to turn off the valve again until the output is full.

And isn't that just a top-up valve? The goal of a top-up valve is to make sure the output side of the valve is never filled to 100% by the input.