Went to change the filter on my RZ dust mask today and noticed a fairly large sliver of light coming though one of the exhalation valves. The little flimsy plastic valve wasn't tight against the seat. I found there was quite a bit of sawdust underneath the flapper and it was preventing it from closing. It's possible it was closing on inhale, but no easy way to tell and judging by the sawdust stains near the valve I suspect it was staying open a bit, which sort of defeats the purpose of wearing a mask.
I took it all apart and cleaned out the sawdust, flipped the flapper disk around and all seems well now. I think what might happen is that the valve gets damp from my breath, and that encourages sawdust to stick to the area and build up. I'll check it every use now.
Be careful when cleaning, the little flapper valve likes to pop off and hide. DAMHIKT.
I wear an elipse mask when I'm doing any heavy dust producing stuff, but there for a while you couldn't get filters for it because, well you know, so I started wearing the RZ more. But you can get elipse filters again so it's back to the elipse for me most of the time.