To check the bellows on my folders (Mockva 2 &5), I bought a surface mount light socket and wired it to six feet of lamp cord. I use it with a 15 watt or smaller builb. To check the bellows, I set the socket down on a flat surface, open the back of the camera, and set it down so that the light is inside of the bellows. Do this in a dark room and any light leaks will be evident.

You can fix pinholes with a mixture of wood glue (Elmer's or similar) and lamp black. I have also had good luck with black fabric paint. In either case, leave the camera open until the glue or paint dries.