The light trap in old wooden film holders can open up due to the pressure from the comb shaped brass spring inside. The spring keeps the light trap shut when the slide is out and it does not need to exert a lot of force to do so BUT the thin wooden bridge across the top of the holder, the thing the comb-spring presses against, will bend. It may take 60, 70, or 80 years to bend but bend it will. Then the light trap stays open when the slide is pulled and film gets fogged.
All is not lost. There is a solution albeit a violent one. Get the right size sharp wood chisel (1" for a 8x10 holder) and use it to crack off the light trap bridge at its seam. I lay the film holder flat on its back and whack into it with a horizontal chisel working from both sides in turn. Take off both light trap bridges. Keep all the stray splinters or chips but discard the tiny nails. Now re-bend the comb spring to cope with the dark slide gap that has crept open over the years.
Re-assembly is merely re-gluing everything (chips and splinters included) back exactly the way it was. I have "rescued" many wooden film holders this way. It's risky but the alternative is a bunch of worthless non-light-tight holders.
Bear in mind that the comb-spring keeps pressing and the holders will eventually lose light-tightness. You may have to repeat the procedure in another 80 years time.
Or use a heat gun to soften the glue and gently pry off the bridges. At least that's what I just did with an old wooden holder that needs the light traps overhauled. I took my time with the heatgun and was able to preserve the varnish on the holder. I also used a sharp scribe to pry the brass up enough to be able to remove them prior to removing the bridges.