We are able to repair the boot with a new zipper! The only correct repair is to remove the old zipper completely and replace teh new zipper between leather and lining. The zipper is never glued, but taped!
We are able in stretching the shaft for about maximum 1,5cm. That means you can't stretch unlimited riding boots. The leather in all his variations, the lining, the place of the zipper determine the results.