I picked up a used G21 today. Pretty dirty inside. When the slide is manually locked back, it takes a very strong thumb to release it, almost two thumbs. I've never field stripped one yet, and this one needs it. I'm wondering if the slide stop may be bent some causing mucho gusto to release it. My Newest G27 just needs a moderate brush to release, but this one is very tough. Any ideas? Dirty? Worn parts?