I'm pretty sure that the check valve is on the vacuum source line. It should connect to one of the rubber sleeves that connect the cross-tube.
Here's a post that might help:
Uh oh... Pulled Intake and now have vacuum lines flopping around
See the valve and where it's connected?
It may not be the same position on yours, but it should go to one of those sleeves, where there is a hole the size of the tubing.
(H) to (L) in this diagram.
