[Geoman]

Might be a silly question, but would someone explain to me how the two tubes that enter into the windshield washer fluid reservoir relate to each other? I understand how the one tube directs water to the pump when the pump is activated. And the output port/tube from the pump moves water through a 3-way connector on the reservoir cap and then to the sprayers.

My specific question is what is the purpose of the "extra" port/tube on the 3-way connector that goes back into the reservoir? I ask this because I'm redoing the system and when I checked out whether the 3-way connector was clear, it appears that there is NO connection in the fitting between that "port" and the two other openings. Is it a check valve?