I recently installed a new Raritan PH Super Flush Head. I am very happy with its performance, but I noticed that randomly there is a sea water leak from this plug (pictured below). Originally I noticed it when the black water tank was full and I realized I had a clogged vent line, but the vent has been fixed and flows freely. Now the tank has been pumped and I still occasionally see seepage from the pink middle of the plug. I figure it is some sort of relief valve but I can't figure out why it is sometimes seeping sea water into the head. Any thoughts would be appreciated.