[jcslocum]

The valve opens and closes while driving under pressure. Keeping it UP for a few days allows the air trapped to move up into that tank. Parking the car rear end high is helpful. It's a big loooong system that needs time to burp.

You need to know if there is a thermostat in there. It slows the flow of coolant down so it can shed the heat to the air. No therm can equal overheating when revs are high.

Have you cleaned in front of the rads between the AC condenser and the rads??