It's not a light leak in the back of the camera..... that would fog the relief of the film. It's an issue with the shutter. You may need new seals around the mirror chamber. When you say 'the shutter is working perfectly', does that mean it's been tested for light leaks? Or are you assuming that because it goes 'click' when you push the button it's functioning as designed? Have the shutter blades / curtains been checked? My guess is there's a leak coming in around the shutter from the mirror box.