An overflow tank will work on an unpressurized system if the overflow tank always has enough coolant mix in it to keep the hose going into the overflow tank submerged in the liquid. I've got a 37 radiator that is not pressurized and the overflow tube actually runs out of the top of the radiator tank. The system works fine. To keep a stock appearence I hid my overflow tank down under the front of the truck. It has been working great for about 12 years!! Let the hose coil around in the bottom of the overflow tank and be sure to keep a couple of inches of coolant in the bottom of it and you got it!

"Truckin' Around .......... Since 1937!"
My name is Joe and I am addicted to Classic Country Music. I just can't hep myself.
Operators are standing by to take your calls!
Now cruising in the Passing Lane