Won't take responsibility for this, because I have Broken zips doing this, but I find the only way to get them to start working again is to use a pair of pointy nose pliers and gently squeeze the sides of the zip closed a bit more. Does make it firmer to zip but doesn't leave it unzipped behind you. Just recently had to do it on my 10 year old engel transit cover, works great now, just a bit firm.
Lemo.
Lemo you are the winner.
I brought some silicone spray and gave it a good squirt, run it open and closed a few times and while it certainly opened nice and easily it didn't make any real difference when closing it up.
Out came the pointy nosed pliers and I gave it a squeeze at the larger end of the zip on both sides. Oh no. The friggin thing wouldn't zip more than a few inches without coming open.
Had a good think about it and squeezed the back end of the zip on both sides (had to have a bit of the zip open behind it to allow this - not a problem as it wouldn't close anyway)
What do you know, just like new, especially with a squirt of silicone to slide on.
Thanks to everyone, especially Lemo.