Fixing bellows leaks can sometimes be easy. Go in a dark room with a flashlight and your camera with bellows extended. Shine the flashlight through the open back of the camera, and look for light coming through the bellows. It usually happens at corners of folds.

You can sometimes fix it using 'tool dip' or liquid electrical tape. Dab a bit on the hole and let it dry.. I've fixed bellows pinholes this way before.