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This is purely speculation, based partly on troubleshooting badly abused cheap cameras in a school environment - TVEI Photography probably didn't make it to Berlin...
With time and use, the seals will suffer permanent set, i.e. deformation that does not recover when pressure is removed, which will compromise it's ability to conform to the lip around the edge of the back.
If the back is distorted by having a twist to it, modest hand pressure could hold the top edge in full contact with the seal and allow a gap to open at one side of the bottom edge. Similar pressure at the bottom edge would cause this gap to close but a similar sized gap to appear at the top edge but on the other side - imagine the back rocking on opposing diagonal corners. Potentially there might be a mid position where the back 'just' seals all round, and any light leakage will depend on how you are holding the camera and the temperature of the back and the body (curvature of ribbed plates being a function of thermal expansion - had any hot days this year?)Some play is to be expected in the hinge, but it might be worth opening the back and looking for signs of the free edge (side with the catch) being at an angle to the axis of the hinge pin.
If you're going to run a test roll, try applying deliberate pressure to the back at top left, then top right, bottom left, and bottom right before taping one edge at a time - and keep notes on what you did, in what order, so the results can be interpreted correctly and you gain more information than 'it leaks' or 'it doesn't'.Equally, I could be talking a load of (overly verbose) rubbish...
MrE-
Thank you very much for your input, it's appreciated!
I'll see if I can be arsed to really figure out the leaks origin, or if I just tape all the things.
After all I bought that body (Nikon EM) for less than what I pay for a roll of film & dev.Ironically I just scanned a roll from my other Nikon SLR (Nigon FG), that already gave me grief as the shutter "coughs" like it's dying soon. Now I realized most speeds just don't work - when set manually; when in aperture priority mode they seem to work perfectly.
FFS.
salad
Thanks for your feedback!
Did inspect the back again.. seems really okay though there's (very) little play on one side.
It's an old but cheap Nikon (EM) - albeit bought in mint condition, delivered good results so far, has never been dropped etc. and always been kept in a padded bag.
Guess I'll have to tape the back and shoot a quick cheap roll through it so see if it leaks from there.
But why do the leaks appear neatly on the left edge of the frame, then go away, and then re-appear on the right side of the frame??