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:
http://986forum.com/forums/324446-post2.html
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.
