I seem to have problem with working out how valves work. There are four types, with one-way being most obvious one, but there are another three valves which controls flow of fluids. From my understanding of descriptions and ingame observation I found some strange behaviour of valves.
Priority valve allows for flow from input system with over 25% pressure to output system. This seems to be working... mostly. From what I've observed it might be counting additionally producers and consumers in pipe system, but I see it working when pipes are around 40-50% full.
Top-up tops up output system up to 50% (pressumably) from source system - hovewer it doesn't seems to be true. This simply makes flow go one-way from source to output.
Relief valve should allow for flow of fluids when source is 75% full, hovewer I cannot see it working. Source is completely clogged up, yet the flow doesn't work and output is zero.
Is my understanding of valves flawed or there is some kind of bug? They don't seem to work like they work in other mods, or the description isn't accurate for what they are doing.

"Pressure" in this mod is a little unorthodox, but it uses the tools that the Factorio game engine is capable of to create a system where higher quality pipes have greater throughput. The main effect of pressure is that way more fluid per second will travel through a pipe 4 than a pipe 1. A vanilla pipe might fall somewhere between pipe 1 and 2, depending on distance.

The valves don't operate on how full their source is, but on what the pressure is. Pipe 1 goes up to 40% pressure when completely full. Pipe 2 is at 60% pressure when completely full. Pipe 3 is at 80% pressure when completely full. Pipe 4 is at 100% when full. Tanks can handle a little more pressure. A medium tank 1 goes up to 85%, 2 goes up to 90% and 3 goes up to 100%. 100% of what? An arbitrary amount that the best pipes can handle. No point trying to quantify it with units, since it wouldn't be entirely consistent depending on what fluid was in it.

So since pipe 1 and 2 never achieve 75% pressure, the relief valve is not usable with them. However, even tank 1 can go up to 85% pressure, so in the early game you can use a relief valve on a tank, but not a pipe. The trick is that you won't get a tank fully full if you just hook it up to a pipe 1, because of the pressure difference. Since pipe 1 has only around half the pressure rating, it will only make the tank half full if you hook them up directly. But if you fill the tank with a pump from that pipe, then it will fill all the way up, to over 75% pressure, so the relief valve will work. This is the reason that on the tech tree, you get relief valves at the same time as pumps. They're unusable in the early game without pumps.

Assuming you're hooking valves up to pumped tanks (or use later game pipes with 80% to 100% pressure), then the purpose of a "priority" valve is not as a low pressure makeshift relief valve, but so that you can have a fluid system prioritize one output over another, only going to the the lower priority destination if you have enough to satisfy both. It is intentionally lower than the other 2 valves so that you can use all 3 types of valves simultaneously for different outputs on the same fluid system. In the very early game before pumps though, it does serve a double purpose as a primitive relief valve.

Ah, I see, so there lies difference. This does clear up things. The pressure system does explain why fluid tanks won't exceed % pressure of the full pipes 1 and 2 without help of pumps. Thanks a lot of quick response! :)

I don't know the best way to get that information to people. It's way too much for a tooltip on each item. I could look into those popup help pages, but I don't read those, so I don't know if other people do. I'll at least try to add something to the FAQ here. You're definitely not the first person confused by this system.