Snakebit .... way to go ! Unfortunately , trash left over from machining operations doesn`t always show up on the test stand ... and sometimes the cleaning equipment doesn`t get everything . I had this problem on some cylinder lock valves from one of our plants . Makes a miner real unhappy when the roof shields that are supposed to protect him don`t stay up . In my case , the seats weren`t machined correctly and the wrong springs were used . luckily , no one was hurt
... but we did have to replace 200 valves in the field .... and all of these valves were 100% tested as I verified .