I have been using the Fuji PA-145 instant film back (same as Polaroid 405) for my view camera and I had some troubles with seriously overexposed photos. This back barely fits on my Sinar back due to the tabs it has on its right side. So, what I discovered after getting three almost white prints, is that there’s a light leak from the right of the back.
I can’t push further the back (the dark slide prevents me from doing so), so a very simple solution was to use my dark cloth to cover the right side. After that, photos came out fine (see image below).
(c)2013 Konstantinos Besios. All Rights Reserved.