I had a leak very much like this on one of my Deardorffs. I chased this at the holder level until I almost went nuts. The light leak ended up being pin holes in the creases of the bellows folds.

There is one way to check this out for sure. That is to take a holder and place in the camera with darkslide in place. Remove the lens board and place a flashlight into the camera cavity with the bellows extended. With the lights off in the room, begin shining around the interior of the camera and if the leak is at the camera back or the bellows you will see light. If you see light, you can be assured that it will leak light under exposure conditions.

You can also check your holders in the same way. Remove the dark slide from the holder and shine the light...check the trap and the hinge. If you see light the holder will leak. Good luck.