Just drop a loose fitting, large head nail into the top of the overflow tube!!!
On a cold engine, just enough water to cover the tubes AIN'T enough! Think about how much capacity you're losing from a tank full of AIR!
Theoretically, Henry's high mounted overflow tube "should" maintain that high level, even after some is dumped out the overflow, due to expansion. BUT, with the water really churning around, at speed, "splash over" into the overflow will gradually lower the water level, until it gets HOT! Just look in the radumator & rev er up & you'll be SURPRISED at how the water churns around in there!!! THE OLD, SIMPLE NAIL THINGY KEEPS THE WATER IN THE RADUMATOR, WHERE IT'S SUPPOSED TO BE! They ain't AIR COOLED! Worst thing in the world, is to have a cooling system FULL of air bubbles! If one of you would splice in a piece of clear plastic tubing into the top hose & rev 'er up, you'd see what I mean.
I have a 2 quart overflow tank, hooked to Vermin's overflow tube & with that, he will maintain the tank level clear up to the top of the overflow tube at the filler. Filler caps MUST be sealed totally, even if you have to take off your ART DECCO goodies & go back to a stock cap.
As soon as I get an early Volvo expansion tank, with a 5 # cap, VOILLE, Vermin will have a pressurized system! My contribution to our EXTREME DROUGHT CONDITIONS, HERE!
Bill W.