If you have to yank pretty hard on any zipper, it's going to break after regular use. My buddy just had the zipper come apart on his Dainese leathers, which we all know aren't cheap. He is just a bit errrr... grand for his leathers at this point and the metal just became uncrimped.
Your best defense against worn zippers is just to buy gear that fits well. Make sure that you have both the instep and calf adjusters pulled alllll the way out when you're putting on your boots. Make sure that your leathers are hiked up, too, since adding in all that material will make the fit even tighter.
Even if you don't think the warranty is still valid, it couldn't hurt to ask.
