Yes, that horizontal line is likely the seal where the back opens, up toward the ISO dial/latch.
See my PBase SQ-A page, toward the bottom. The shot labeled "Oh my Back!" is from a back with practically no seal left. It's the film coming off the supply spool that gets hit, not the film in place at the frame mask, so it's appearance can be variable depending on ambient light and how fast you wind through frames.
The good news is it fixable!
(Late catching up with this, having no power at home since Monday night.)
I finally sent it off to Koh's Camera and he fixed it perfect for a crummy $20. Wish I hadn't waited so long. He's the Bronica guy.
Update: I fixed the light leaks for the most part, but since 95% of my shooting is done in cloudy/rainy weather, I figure it wasn't much of a test.
I recently did a lot of shooting in intense sunlight and found a good portion of my negs ruined by light leaks. This is just one of many photos afflicted, all of them with a leak of the same intensity and location.
I'm trying to figure out where to locate this particular leak so I can go in and add some more foam. The image is flipped upside-down, correct? So based on this the leak is coming from the center, spanning downwards on the left side of the back?
Hmm. I wonder if the seal across the hinge part of the back is gone/deteriorated? (#3 in this shot)
I think the leak is stronger toward the right side, viewing the camera from the back when the film is upside down -- but it could just mean the sun was on that side!
These things can be weird, the actual leak may be near the supply or take-up spool and not striking the actual mask frame area. In such a case, it's either the frame before, or the frame after the shot that gets light struck. My hands-on personal experience with these problems is still fairly limited (thank the Flying Spaghetti Monster!) so it's difficult to speculate on the full range of possibilities.
Another dubious thought is that possibly the light baffle behind the mirror in the body isn't closing all the way, and light comes in through the lens and around the sides of the mirror. As I type that it sounds far fetched, but obviously something is not in factory spec.
An additional vague possibility (based on analysis, not experience! :blink: ) is that the back isn't seating tightly due to the little retaining hooks being worn or loose or something.
I suppose black (gaffers') tape could be carefully applied to help in isolating the light path. If all the back attachment and closure stuff is taped and it still happens, that might suggest the light baffle. If the joint between the back and the body is taped and you still get it, it pretty much has to be one of the long seals at the top or bottom.
You could also plan some real test cases -- put the camera in the sun with a lens cap on and pretend to do some exposures, change the angle of the light hitting it, etc.
That's what passes for my thinking -- no charge! :D
And the next day ...
I've been thinking (smell the smoke!) the fact that there appears to be light struck area on the film rebate pretty much means it's a seal problem. The frame mask is between the film and the body shutter, and even the dark slide and the back to body joint. The rebate is more open to exposure between the spools and the insert rollers.
There -- how's that for profound pronouncement!
They are really neat and capable cameras, but the back light seals appear to be a weak spot. And of course virtually any backs around are at least a dozen years old -- and maybe thirty!
What exactly do you mean by "rebate?"
So you think the leak can be located somewhere around the spool of the back, and not the darkslide slot? That certainly helps!
What type of cheap film should I run some tests with? If this was 35mm I could just run to the drug store and grab something cheap...