I know that almost every "active component" (aka everything that is not just a "pipe") has a small internal storage for fluid and gas. And while the name suggests otherwise, the fluid relief valve actually lets gas into its internal storage volume. It does only let fluid through, so the basic function of the valve works, but this can still be an issue and i'll explain why.

First, to demonstrate/reproduce this, simply spawn two prefab tanks. one with nothing attached to its ports and one with a single fluid relief valve attached to one of its ports. then compare the mouse-over tooltip of the two tanks.

You'll see that some of the gas the tank spawned with has been transferred into the fluid relief valves internal storage (e.g. compare the amount of nitrogen in both tanks, the one with the valve has less nitrogen). If you have a large custom tank with many ports and relief valves the effect is exacerbated, since every relief valve contributes to it.

the gas relief valve shows the same behaviour as the fluid relief valve, it unexpectedly lets both fluid and gas into its internal storage.

why is this an issue? Because in certain situations you never get the content of the internal storage "back". Let me elaborate.

Consider a tank with an "outlet" port (that may feed an engine or some other "consumer" with fluid from the tank) and an inlet port to refill the tank. there is a fluid relief valve on both the outlet and the inlet port, so that only fluid can ever enter and exit the tank (so to never screw up the internal pressure the tank spawned with).

if you let the engine/consumer run the tank completely dry, you'll find that the final pressure in the tank is significantly lower than it would be if the inlet(!) port didn't have a relief valve. The reason being that during the spawning, both gas and fluid were transferred into the internal storage of both relief valves. This gas and fluid is now unavailable for the engine since once filled, the internal storage can never be emptied/recovered. The gas that now sits in the internal storage of the fluid relief valves is "missing" from the tank and the reason why the tank ends up with less pressure than expected.

And like i said, every additional relief valve on the tank exacerbates the effect.

my suggestion: dont let fluid enter the internal storage of the gas relief valve, and dont let gas enter the internal storage of the fluid relief valve. Whats the point of letting it enter if you don't let it through and never give it back out?

I understand why the components in general need a small internal storage and i don't argue against that! but there is a certain irony that the very components designed to avoid screwing up pressure in tanks contribute to screwing up the pressure in tanks ;-)

So in the special case of the two relief valves i would argue to let them fill their internal storage only with fluid or gas respectively. I can't see a way how this would "break" existing creations, but IMHO it would certainly help to simplify tank management.