Duckbill Check Valve 1/8" MicrolineThis part is located under the manifold.
You will need to remove the Membrane Housing and Membrane to see where this very small part is installed.
It is held in place by a small retainer.
Be careful when pulling it out, making sure not to accidentally push it inward.
If this small item is stuck CLOSED, then no water will flow through the membrane to the storage tank.
But also check the ASO diaphragm ( more common part to fail than this Duckbill check valve ).
And most "lack of water" from the unit, is caused by loss of air pressure in the Storage Tank.
Note: Do not use any lubricant on this part when replacing.