I was thinking about filling my caster tires on my walk-behind with Great Stuff (that super-expanding insullation foam at hardware stores). First I would remove the valve core and then fill the tires, allow it to expand, and replace valve core (possibly inflate to increase pressure). My hope would be that this would make these tires completely flat proof. I realize that this is either a Great idea or a terrible one. P.S. I have already tried fix a flat with no improvement on one tire.