I can't be sure but I believe what happens is that the bushing at one end wears down and the valve starts to hit the sides until it breaks (found pieces in the plenum as well as the cat converter). The valve that broke still had the screws in the shaft and the butterfly valve next to it was worn on the edges from hitting the sides. In the used plenum I bought the shaft was loose and I had to rig up a new bushing with a copper compression fitting. I'm still not at ease with the whole system and when I rebuild I may take the valves out completely, not worth the performance increase versus the chance of damage in a 16 year old engine.