When the postman delivered my Bronica backs first thing I did was checkout the seals and brush out any debris. Next I load up some arista 120 from Freestyle (cheap for testing) then I put the back on the camera and rotate the camera under direct sunlight then start taking some test shots. Any light leaks will show up. I do the same with any new camera.
The only one I had a problem with seemed to have a bent cover or messed up latch. When I closed/latched the cover it would pop back out a little. It created light leaks that were really close to what you have been seeing.