On or around 7 Oct 2005 15:40:55 GMT, Derry Argue enlightened us thusly:
cast iron is softer than steel caliper backing.
We never imagined it was. But I think the fact that you said you cleaned about ½ cwt. of sand out of it points to the cause of failure. Previous maintenance wasn't adequate for the operating environment, I imagine.
Mind, it's a valid point that the front calipers with 2 pins through the pad wouldn't suffer this kind of failure.