Light leak in a Bronica SQ-Ai back -- advice needed
I have a Bronica SQ-B with its original 120 back, plus an extra 120 back for an SQ-Ai. I've used both without problems for several months, but the last two times I used both, the developed film from the SQ-Ai back showed some sort of light leak on every frame. The film from the other back showed no leaks.
The leak appears in the same spot on every frame (upper left), but varies in intensity, sometimes quite noticeable, sometimes barely . It crosses the rebate. Here is a scan:
I checked the back carefully and all the light seals are fine. Plus I can't see any tiny holes or cracks that might be causing this. The condition of the Sq-Ai back appears identical to the other back that is light-tight.
I doubt that I let light into the film roll as I was loading or unloading the insert (twice), because the leak appears on exactly the same spot on each frame.
I'm really stumped. If anyone has any ideas about this, I'd be very grateful.