Door lock check valve on 300cdt 1982
 

Should there be a door lock check valve attached to the baby blue rear seat-back vacuum line just in front of the firewall on the driver's side straight from the 5 plug connector? It looks like maybe PO did a hack job, as the 2nd of two outputs on the check valve is plugged with rubber cap. Do I need this valve or is it unecessary and perhaps the culprit to the rear seat-backs not locking? Thanks for any input.