Igor, the only way to cure the leak is to locate exactly where the light comes in and then block it. To locate the it, first check if the camera remov. back is bent or distorted, check also if the foam or whatever material is used to make it tight is missing or distorted. To do it, get a small flashlight, wrap a decent amount of thick black rubber foam around it and remove the lens, next place the flashlight in the lens mount (the foam must be enough to hold it in place). Go to a darkroom (or whererever you no light coming in), put the shutter on B, turn on the f.light, and check with back in place if there is some leak. Allow ten minutes or so to get your eyes used to the darkness so you can see the leak. If you see nothing, check with back removed and shutter closed if the shutter metal curtain produces the leak. It might be a very tiny hole in it, hopefully not. Only cure available is to replace curtains, it is major surgery. May be some other remedy of which I do not know yet. If the foam is missing, you can either locate a suitable material to replace the original, or (I did it 25 years before) locate some wool thread (black) place three/four threads twisted in the slot and stick it with contact cement. Apply very low amount of cement unless you want the camera back permanently stuck. AFAIK, this is still OK and made the camera light tight within half an hour, and lasts today in good condition.
Hope you solve the problem. Good luck!
