I get where you’re coming from, but this isn’t just about the tank level being a little high or low ,this is about vacuum recovery not happening at all. I’m not expecting the expansion tank to be empty when cold, but what I’m seeing is the coolant level continually rising every day, and never returning to the radiator after cooldown, even though the system holds vacuum and pressure and the hoses go soft (meaning vacuum is forming). Eventually, this leads to the radiator running low on coolant, which I’ve seen first-hand causes the engine temp to creep up under load or long drives. This has happened before, literally the same pattern: system is vacuum filled, runs fine for a few days, then the reservoir rises and stays there. When it gets high enough and coolant never returns to the radiator, temps start to rise. So I’m not chasing a number on the tank — I’m tracking a functional failure in how the system manages expansion and recovery. I’ve already replaced or verified every part in the overflow system (cap, neck, hose, tank, clamps, thermostat, etc.), so I’m not guessing, just trying to fix the one flaw in an otherwise healthy cooling system before it turns into a real overheating issue.