I have found that when I leave p-brake on and let clutch out it shears all 4. They are usually really easy to back out, a center punch and a little tapping or just spin em out with finger or remove housing, send to Georg & put new one on, remember to save the shims and it wouldnt hurt to check...
Since it is so clean now would be a good time to put some heat shield on the bottom of that, it will reduce heat & sound , it's self stick, has semi rigid metalic surface and is flexible so it will conform nicely to that piece. I think wngrog posted a link in the light blue 40 rebuild thread.
Shackle bushings worn, loose will cause this. I'm guessing your shackles are straight up & down ? What deg do get when you put angle finder across upper trunion studs with truck sitting level ?