[Blas]

Assuming you have a Superformance, and have the usual arrangement… (Your SPF car number would help here)…
And just so we have our terms correct here; The Expansion Tank is usually connected to the engine and then a big hose from it goes to the radiator. The Overflow Tank is connected to the Expansion Tank via a small hose and may only have a small drain valve on the bottom.
The staining on the overflow tank is probably just some excess coolant – there probably is e a small vent hole drilled on the top of the overflow tank where it came from.
Do you have the black painted brass expansion tank or the stainless steel expansion tank?
If it’s the black one, the usual problem is fatigue on the bottom hose connection to the tank because of the vibration of the engine and poor original design of the tanks bracing, I’ll just bet the tank brazing is cracked allowing more vibration on the tank flange. Look here first for the problem.. The stainless tanks have had very few problems. A good radiator shop can refurbish this Expansion Tank quite reasonably. The SS tanks cost major $$$.