Are you using one of these?
Do you roll this bin around frequently? Also, how do you store clay inside - do you mix it and just deposit it inside, or is it bagged up?
If you're "digging" out your clay from a walled tub, that's gotta get old real quick - I'd suggest you get a new system that allows access to the sides of the clay. Could be as simple as a platform/furniture dolly with a big storage bag sitting on top (that's what we do, only it's scaled up to 4ft pallet size)
Consider ditching the casters and let it sit on the ground (or mounted on a pallet) and move it around with a pallet jack. Heavy ceramics is going to need a pallet jack at some point, might as well get one.
Otherwise, try and fabricate a new base from metal and put the best casters you can afford under it, maybe even 6 casters instead of 4. Steel wheeled casters tend to hold the most weight and guess what, they don't flat spot. I can't see it rolling on its own unless you have sloped floors, so no brake is necessary, just wheel chocks. Overkill is always best, so definitely make it hold 2x your intended mass.
We use trash cans to store reclaim clay - they get rolled around dirty floors, cracks and seams in the concrete, etc and hold over 350# each I'd guess. The caster base is VERY simple and could easily be fabricated to fit a variety of containers. Casters can easily be swapped out to a different style/brand/size/etc. The ones below are at least 30yrs old and going strong.
I've noticed in general that the Rubbermaid/Brute tubs don't really hold up due to the weight of ceramic materials and because they flex. We do have a few rubbermaid cans currently, but you can see they're no longer on the bases made for them. One shallow nick or cut on these cans usually ends up in a replaced container within a year, that's why the rest of our cans are metal and going on 40-50yrs. The only issue with the metal cans is over time the galvanized metal can possibly flake off. A good alternative might be a thick rubber livestock water trough/stock tank.