UPDATE!!

Problems solved. I wanted to come back and let you guys know what happend.

It turned out when I installed the new expansion tank, I made a mistake. Well me or shotty products, who knows. On the bottom of the ET there is a large opening that has a rubber seal, which is held in by a conical piece of plastic like a washer. On mine, for some reason, this plastic piece fell down slightly and covered the grooves the wire clip slips into when you install to hold it onto the hose. This cause the expansion tank to basically just be sitting there and not clipped in place. It was in there good enough to not cause issues but resulted in a slow leak. I couldn't see anything just laying under the car until I added some pressure with the pressure tester.

Corrected this, reassembled, added coolant, bled, checked oil for coolant or discoloration (just in case), and all was well. I feel like an idiot but i'm happy now it's all done!

Thank you everyone for the tips/tricks/help.

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2004 E46 M3 Silbergrau - Mein
2004 E53 3.0i TiAG - Wife's
2002 E46 330i TiAG - Sold
1999 E46 328i Schwarz - Sold
1961 VW Beetle GolfBlau Ragtop Sdn - Mein