@philipbrewer Even our much smaller dogs were quite strong. Have you thought about using an alternative leash, like the Gentle Leader, I think it's called?
@Miraz We bought a Gentle Leader, but were never able to get it on the dog. :-/ What we're using is a Martingale collar, where the leash goes once around the dog's chest and then back through a loop in the collar. When the dog pulls, friction across the chest absorbs a lot of the pull.