What is the check valve for?
The check valve prevents water from going back into the membrane. This part can be replaced easily. Based on the age of the unit, they may look like one of the two pictured below (inset):
To verify that the check valve is still functioning properly, you'll need to unscrew it. Due to its close proximity to the other (waste line) fitting, simply remove the collet first, then unscrew the fitting counter-clockwise until it comes out.
Once the fitting has been removed, blow on the end that the check valve is in. Air should flow through it. Blowing on the end (where the tubing was inserted) should not allow air to pass. If air only flows one way through the check valve, it is working as it should. Reinstall. If you need a new one, contact me and I'll be happy to send one out immediately. This part is available in the Reef Shop on this site.