I didn't see sand until I back flushed the heater core. It seems the heater core trapped most of that stuff. You could probably get a lot out (if any is present) just by draining just the heater core. That way you don't have to drain the whole system again. I used my garden hose to do it but you want to be careful not to apply too much pressure to the core. The hoses come off really easy too.
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Excellent! It’s now my weekend project.