with condensation under the filler cover..... it definitely sounds like the o-rings are bad. The entire valve could be bad but doubt it based on the valve's pressure thresholds (see pressure test below). When the entire valve is bad, the results are much like having a bad coolant cap and you end up w/ what's affectionately known as a peeing Boxster. When this occurs, it dumps almost the entire cooling system contents in one shot after engine shutdown. It'll be plainly obvious by either the rushing water sound you hear or the extremely large puddle in front of the right rear wheel
Anyhow, some dealers may try and tell (thus sell) you the entire tank needs replaced....... but the o-rings and valve assembly are available separate [from the tank]. Sunset or Suncoast sells them in case you don't have luck locally. Here's a write up on how to replace the o-rings:
Coolant Tank Bleeder Vavle O-Ring Replacement
Incidentally, if you ever do a pressure test on the system, you are to pressurize it to normal operating levels - 19psi. The bleeder valve is designed to open at 20psi and close at 14psi. So at 19psi, you'll be below the threshold of it activating and can truly test the system for leaks. I wouldn't worry about having a test done unless your problem persists after you replace the o-rings.
Good luck
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