You can test the light traps by pulling the dark slide, and shining a flashlight at the trap, and observing from the winder side, looking into the trap area. Done in a dark, or dimly lit room, it should tell you if the trap is leaking.

There are several guides on the Internet for making your own.

The problem with mine, original 12 backs, was the counter would not advance to the first frame. Beautiful pristine backs, but 40 years of non use required a full CLA on the backs.

Sent from my iPad using Tapatalk HD