I got into a 3400 chain brake over the last few days because the brake band was dragging badly. I've never really given them a second look before. After looking at it and trying to figure what was wrong, the pawl (trip lever inside the cover) was worn. The further it wears, the more the band tightens up. I did manage to find a PN in the PP375 IPL but it's NLA and likely as rare as hens teeth.
I made one up and it seems to work fine. I need to make another. I'm not sure how anyone would wear one out.