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This is an issue that just popped up today with my rear brake line; whenever I snap the brakes, this thing pops out; the brakes still work when it happens but it seems like a safety issue. If I apply the brakes gradually like I usually do, there's no issue, but if there was ever a reason I had to come to an immediate stop it could be a problem. Only the rear brake line does this, not the front.
Pic related, took a video and converted it to a gif