IF someone had the T-20 apart, and IF they didn't get the band anchor pin back in correctly, then it could cause the plunger to lock in it's bore. I can't see how this could happen in a T-20 that has never been torn down, but lets not rule anything out. If the pin is out of place enough to lock up the plunger, the case half is probably toast as the steel pin will have destroyed the aluminum bore. This is typically only an issue when rebuilding one. If you don't get the pin seated correctly, or if you spread the tabs in the plunger that the pin rides in, it can cause a plunger to stick. what I suspect has happened, is that you've pushed a plunger in too far, and the O-ring is preventing it from coming back out
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