I have been plagued by light leaks lately... Always along the long side of the film, toward one side. Some holders have been worse than others, but all have given leaks at some time - and not at others. When I shot the solar eclipse this spring there was no sign of it, despite the dark slide being out for well over half an hour - in sunshine. I was inspecting the bellows yesterday without finding anything. But then I suddenly realised that on the back of my camera (under the ground glass assembly), there were two strips of velvet. There was none on one side of the back... So now I'm taking my camera up to the local shopping mall to buy three narrow strips of black velvet; I'm replacing all to ensure even thickness.