It's a one way valve with a spring and a rubber seal. Trust me. I took mine apart when I put on the turkey baster version.
i agree with you, i didnt know it even had a spring though. I thought after enough psi build up it would be able to push up the little metal cover and escape. When it cools down itll suck it back down. Mine had some pshhhhhh when i took mine off. I installed in on my bobs tanks and blew through it and it allowed air to go through. Its definitely not "easy" for the air to go through but it lets pressure out.
So theres really no way to get all the pressure out without having an open filter and that would get messy.