The hammer moves barely a few mm... as much as i'm loathed to contradict Pete, I think you're feeling the air leaking past the valve stem. I've taken mine off and just taped over it. There's not enough pressure to blow the tape off. And any debris finding it's way into that area stands a good chance of being stuck to the tape. Plus it's one less thing to go wrong, or let water etc in...
I'm guessing the leak % is fairly consistent. As you say, free consistent movement is key
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