Jeff, Do the zipper teeth pull apart after you pull the zipper pull shut. If this is the case and you have a "YKK" style coil tooth zipper, it is often that the PULL is wearing. The height dimension inside the pull has worn to become too big. This metal pull can be gently squeezed, at the back end or following end, with pliers. Put your plier jaws behind the pull tab on the zipper pull with the pull still on the zipper teeth. Squeeze very gently a little at a time and test. I've worked in a couple mountaineering shops and more times than not I could get a few more weeks of use from the zip on customers gear, our rental gear and my own. Eventually the zipper needs a new pull or complete re-placement. Good luck. Or maybe now is the time for a Springbar, OZ or Bibler!
L43dean-1968 Fj40 LandCruiser,1980 Rn47 Hi-lux Pickup 4WD and 1985 Rn65 ExtraCab Truck 4WD. YEP!