I leave the dark slide out of the film back. Seal at max expansion.
The dark slide, when installed, compresses ever so slightly the rubber gasket. At some point, the gasket gets tired, hard with age, doesn’t expand after dark slide removal, causing a light leak.
When in business, I had several Haselblad cameras and changed camera rather than film back. Another person would reload and film back doesn’t need to be removed from the camera for that to take place. The day moves too fast. I concentrated on what’s happening in front of the lens.
I’ve nrver had a light leak and never replaced any seals. Some of my cameras are from the late 1950’s - early 60’s.
Maybe I’m due for a light leak issue!
Predictions are hard, especially about the future.”