Originally Posted by
terraphantm
For anyone who has doubts about the CDV, try this:
Remove your CDV and clean it really well
Blow into both sides
You'll notice that the return direction has a ton more resistance than the release direction. If you can feel that much resistance with your breath, there will definitely be a very large pressure drop in the actual system too. The reason the pressure drop is only in one direction is because the CDV has a mechanism that allows fluid to go around the restriction in that one direction.