welcome to PI
first question...Is your coolant original configuration or has it been changed.
the original has the "swirl" tank with the radiator pressure cap and the "overflow" tank with just a flat cap.
also, I was thinking the FRONT should be raised to get the air to the front of the engine block and thus up and out to the thermostat.
besides filling the swirl, while running, the top of the radiator is vented
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