When the hook slides down the broken zipper, it makes locking up and down difficult. A good tip to help you easily handle this problem is to use a paper clip, keychains or even use a phone cord to make a sliding hook. Just slide it lightly through the sliders and you can have a new zipper. Surely this is not the most " fashionable " solution, but at least you can pull from inside or outside the zipper easily.
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Having fun!