If you are talking about the valve itself and not a hose connection, the valve should be open when there is vacuum in the manifold and seal like a check valve when there is pressure. If it leaks during boost, some air is going to be blowing into the head. If you don't have a catch can it might mean moving extra oil through the IC and maybe causing knock. Most likely you wouldn't notice a difference. Lots of engines with a little excess blowby run fine.

Still, I'd want a valve that seals. It is easy enough to test - just blow through it.

 

I donīt think so. Its like a normal leak in your system...