Ok, I just got home and will perform a compression test just for the hell of it. While Im at it, Ill use the compressor to pressurise the cylinders to see if it bubbles back into the expansion tank.

The thermostat looks new as I inspected it when the upper rad pipe was off. Didnt think a bad thermostat could keep the system pressurized.....

Edit: 180-181 PSI across the board. and cheepo radiator cap. Ill go to MB in the morning and get a new cap and thermostat.