So, I found a coolant leak from the rubber cap near where the lower heater hose attaches to the block. It was dry rotted, thankfully I caught it while it was just dribbling coolant every now and then. I replaced the rubber cap and ran the engine with a radiator funnel until it got hot, let it cool, and then topped it off.
I went for a drive to make sure everything was okay and ended up puking a bunch of coolant everywhere. The was radiator only ~2/3 full after I let it cool down and opened it up. It looked like it had exploded out of the overflow bottle. The good news is that I don't see any leaks anywhere. I checked the thermostat and it starts to open at 180, but doesn't get fully open until around 200. Not sure if that's normal.
Anyways, thoughts on what could have caused this and how to avoid it? Maybe there was still some trapped air in the system? Also, I'm planning on just replacing the thermostat and seal since I have it out. Any tips on where to get one? I've seen a few different types. As much as I'd like to go to the Cummins dealer, they're 90 minutes away and only open when I'm at work. I have other vehicles to drive, so I don't mind waiting for parts in the mail.
Thanks in advance for the input.
-Justin