The expantion tank is old school brass, just polished.
Checking the coolant level when cold is easy, and the cold level is easily calculated. The general rule of thumb for the proper capacity of expansion is 12% of total coolant volume for draw down, and 6% of the total volume for thermal expansion.
You only need to calculate this once, after that you 100% know your set cold level, your hot level and free space is therefore known too, even without seeing it.
The plastic must go!
