You really need to clean the trough that the bolt (cyl release latch) rides in. A better way may be to remove the thumb piece and spray in cleaner from the back side of the bolt. Of course, the best way would be to remove the bolt but there is that evil spring and plunger to deal with. First time I did that job, I did it inside a large freezer bag. You are most likely to loose the spring and plunger while replacing the bolt because it just don't seem to fit without a bunch of fussing.
Good luck.
remind me to tell you about the 2 piece bolt that came in one of my model 15s.