After you're done poking and prodding and flushing, consider installing a coolant filter.
After seeing how much sediment falls to the bottom of the water jacket I knew I'd not be able to get it all out of the block and head. And you'll get more sediment from carbon etc. later. So I installed a coolant filter, NAPA #4070 for cooling systems of about 4 gallons. They also have sacrificial zinc etc. in them. I took a remote filter base off an old 220 Cummins and put a new spin-on element on it. You can buy a remote filter base from Summit or whomever that takes that filter thread and use the number I gave. Or you can do your own research and use something else. I just cut one open after 4,000 miles which was completely plugged with black sediment. Obviously I didn't get it clean enough on the last overhaul.
From my experience with Cummins diesels I've developed a theory that the sediment comes from head gasket leakage over time. Even with all those bolts, the head bounces up and down a little under full load and it'll seep exhaust gasses into the coolant. I used to see trucks from the Chicago run that ran east and west over the Cascades and Rockies come in to the Seattle shop for maintenance. You could tell who'd been pushing them to the max from the streaks of dried antifreeze running down the block below the head gasket. On these I'd look for head bolts with the heads popped off. I found more than one. They'd get a note in their dispatch box.
