Leak in the pressure relief valve

I have a pin lock keg system and noticed I have a leak in the pressure relief valve. I had it apart and now it does not seem like I can get it tight enough. Either that or I lost some gasket when I had it apart. I am pretty sure I had it tight when I put it back together, but this is a plastic part so I do not want to tighten it too much. I am afraid of breaking something. Has anyone had these apart before? Any suggestions on a fix???

If it's the plastic yellow valve, there is supposed to be an o ring on it. If it's not on, it will leak.

Nope, this one is all gray. In fact. I switched the valve with one from another keg and it still leaks.

I have this problem with one of mine as well. A little keg lube takes care of it, but if I release the pressure its often a gamble whether or not it will hold pressure after. The keg lube works as a temporary fix for me. I have no problem setting and forgetting it in the fridge, but if its something Im going to release the pressure on I avoid using the lid with that valve in it.