Overflow Tank doesn't Return Coolant

[Sylvanite]

Thanks fixkick, I think you hit the nail on the head on this. I realized that there was little missing coolant, it had all just migrated to the overflow tank. I'm now pretty sure it's just a bad rad cap (the hose held vacuum good enough to siphon, so I'm guessing it's not the problem).

That being said, could this have caused the engine to overheat in the past? I'm still trying to figure out why this happened.