[2003S]

Just closing the loop on this one (preliminarily, anyway). The tech pressure tested the system, and the release valve was blowing out at very low pressure. He replaced just the release valve (saving his knuckles, and me the costs of several hours of labor), pressure tested the system, and it's back up to spec. He said he couldn't tell if the tank was original, but he suspected not because it's in good condition with the exception of the faulty valve.

Fingers crossed, the next track day will be the true test -- on the streets no problems.

He also replaced all of my "Non-Porsche Authorized Hose Water Coolant" with the good stuff.