My investigation into light leaks is by no means complete but I'm going to post here what I've found so far then update it if I get more information.
Light leaks has been one of the more common complaints of the T981. Specifically, light getting through the shutter. This has been attributed to the shutter being designed for an SLR which would have a mirror blocking much of the light and so total light tightness of the shutter is not necessary. In the T981, with no mirror, sufficient light is getting through the shutter to cause fogging of the film.
When I got my T981 I tested it for light leaks. I thought the problem was one of light getting between the shutter blades. This, I guessed, would cause banding across the image. I didn't find anything like that so assumed my camera wasn't affected. Later, on a few frames, I did notice some light spilling into the top of the frame (bottom of shutter) and causing some fogging at the very edge of the frame and then reducing the further into the frame it reached. There was no fogging outside the frame so this was coming through the shutter but it didn't appear to be coming through the curtain. I could induce the problem by leaving the camera cocked, with the lens set at maximum aperture, the lens cap off and with the camera exposed to daylight. After several hours like this, the fogging would appear.
I did some tests to see if I could find where the light was getting through. I fired a flash into the camera with the lens off and watched the back of the shutter for any light. I did see some coming from the bottom of the shutter.
It was impossible to tell exactly where this was coming from. I thought it might be light passing down the front of the shutter, striking the chassis below the shutter then reflecting back up on the otherside of the shutter. I removed the shutter and fitted foam in all the places I thought the light might get through, including along the chassis beneath the shutter, but it made little if any difference.
Next I dismantled the shutter to see if I could find a problem. With the curtains removed I could see a difference between this shutter and other similar shutters I had dismantled and the difference, could, I thought, be the cause.
On the reverse side of the shutter frame, these is a large area chamfered back from the edge of the aperture. This is necessary in some places as the curtain hinges need some space. However, the degree of chamfering is MUCH greater than on other similar shutters I had seen.
Here's the T981 shutter frame
and here's another from a Yashica
This chamfering means there is a gap under the first curtain when the shutter is cocked.
There is also a gap with the Yashica frame but it's smaller and the Yashica frame is blackened - another difference.
Why there is a difference between the shutter bodies I don't know. Presumably the manufacturing process was changed/updated as the shutters are, otherwise, the same.
I looked at the shutters from a number of other cameras that use this same Copal shutter and, although not conclusive, it did seem that later cameras had the same version as the T981 while earlier ones had shutters like the one above from a Yashica.
I thought I had found the problem and the solution, I thought, would be to swap the shutter to one of the earlier types. Unfortunately, this hasn't been a total solution and the fogging can still occur. But it seems better. To produce fogging as in the sample above, I need to leave the camera with the lens cap off, the lens at f/2.8 and the shutter cocked for a whole day. With the shutter uncocked, there is barely any fogging with the same conditions.
I really don't think this is an issue for me now but I still have an idea how I might reduce it further so watch this space.