Unfortunately I have to agree with nursemike & mdinana, it's time to resole.

I've tried what you're doing but the flexing kept causing the crack to reappear. The crack also caused the boots to let in water. Buying more glue to try would still leave the cracked boots. I believe it is cheaper in the long run to either resole and have the boots last another year or two with new tread or depending on the condition of the uppers and the price of the resole it might just be more economical to get new boots if you can find a good sale.

I personally don't like heel savers on boots. The hard plastic is more slippery and it doesn't let the lugs bite into the ground as well. But that is just my preference.

Good luck with whatever you decide.