I apologize in advance for the length of this...

I have an 02 4.4 that has needed a coolant system refresh for a while. I replaced the radiator ~10 months ago.

Around 2 months ago, the water pump gave out and left me stranded. The belt was shredded with pieces everywhere, but that was the only damage I could see.

I finally got around to replacing the water pump and pulley, thermostat, overflow tank, upper/lower radiator hoses, and belts yesterday. The valley pan was already replaced ~2 years <15,000 mile ago.

After getting everything replaced, I followed the recommended procedure to fill the cooling system and thought all of the bubbles were out of the system. Started the car up and everything seemed fine other than a slight tick tick noise. Took it for a quick drive around the block checking the temp with my phone and Torque and everything seemed good.

The next morning I adjusted the belt tension trying to get rid of the ticking noise which worked. Checked the fluid level again and took it for another drive. This time it was a bit longer at ~2 miles, and I got onto it some. Max temp I saw was ~215 toward the end of the drive.

Pull into the driveway, stop the car, and immediately all of the coolant is flowing out of the engine. I checked under the hood and couldn't see a source. Checking under the car, all of the fluid was coming out around the middle of the re-enforcement plate. There was a small trail of fluid coming up the driveway from before I stopped.

I've refilled and bled the cooling system with just water. The only leak I could see was around the overflow cap. It definitely needs to be replaced and a new one is on the way. I also checked the oil. No foam or milky color under the cap and the dipstick looks and smells good. So I hope it isn't a head gasket.

At this point, I'm at a loss. What else would cause all of the anti-freeze to drain, at once, when the car is turned off? I've checked every hose I can get to. I haven't pulled the re-enforcement plate off yet, but that's my next step. My intuition is one of the parts I replaced is the cause of the leak, but I just don't see any coolant on any of those parts.