I don’t have a lot to report, but thanks to a good suggestion on ffcars.com, I have made some progress in tracking down the cause of the leaking problem. They say the obvious thing to try is often the easiest to overlook.
Basically, I just rigged up something to seal over the opening of the recovery tank, and with just lung pressure blew air into the tank to force fluid through the coolant line and into the degas tank’s flange. It didn’t take much effort to get coolant spilling out again. A human can blow around 2psi, and I wasn’t blowing all that hard so odds are it was only a single psi of pressure in the coolant that was enough to cause a leak. If the smaller diameter hose running into the overflow tank caused even 1-2psi over the pressure when the valve opens, it would leak (and clearly is).
This doesn’t answer, yet, what is wrong with the de-gas tank, or what I can do to fix it, but its a pretty good bit of progress. The odds are much lower now that its steam in the system, or a head gasket leak, or other much more costly problems. I ordered a micrometer from Amazon yesterday, and will be able to get accurate measurements of the dimensions of the flange — I’m guessing I’m going to discover the thickness of the part the cap holds onto is off by a couple hundredths of an inch — enough to seal when it expands with heat, but off enough to leak until then.
I’ll post more tomorrow.