I recently bought a Bessa II and it started to show light leaks after the first couple of rolls. They would would vary in intensity - sometimes strong and sometimes very faint. I put a small but powerful LED light into the extended camera, turned out the lights in my darkroom, and waited for a few minutes for my eyes to adjust. Nothing - the camera was light tight.
I know this post is regarding a Bessa I but I thought I'd share in case anyone was experiencing something similar - it might fix their light leak issue.
My issue wasn't a light leak in the bellows or back - It turned out that the pressure plate had moved so it wasn't adequately keeping the film against the frame gate. So light was leaking into the feed roll when making an exposure (essentially pre-flashing the **next** exposure) as the pressure plate wasn't sealing properly on that side of the gate. The intensity of the leak varied as the exposure varied. I expect I caused the pressure plate to move when I was loading some film, or poking and prodding things as I familiarised myself with my new (and first) folding camera.
Thankfully it was easy to fix - just made sure the plate is now centred over the gate, and using a piece of black electrical tape stuck to the door I made a quick reference to check if the plate has moved from where it should be.