It should just be a,...hole, I think. But the flap may be keeping the rubber base washer from sealing. I think it should go as long as it only covers the hole to the overflow elbow. And looking again, the seat where the rubber washer on the cap mates, does need cleaning and maybe the sanding I mentioned. I found a suitable round-flat bottomed something that fit loosely inside the neck. With abrasive paper, it was easy to make it shiny and flat, Dan. I think I even put a little water pump grease on the rubber washer. It seals tight, no leaks, and overflow moves up the expansion tank as it heats-returns as it cools.