This maybe a stupid question but I have not been able to find anyone that can explain it to me! I have a Model A with a 472 Cadillac, and when I built it I didn't put on an overflow tank. After several months of driving it, my Radiator is low on coolant (of course). My question is: How does the water get back into the radiator when needed? I understand when it gets to operating temp and the water expands and the pressure forces the spring up and the water flows into the overflow. Because it's a closed system, when the water cools down it contracts and creating a vacuum. How does the coolant get past the spring loaded cap??????
You have to have a radiator cap with a second valve that lets vacuum draw water back from the tank. It is a little disc type check valve in the bottom center of the cap. I'm guessing most radiator caps you buy now days has one.
Thank you that makes perfect sense. It amazing the amount of people that have no idea and thanks to you I am now not one of them!
Before recovery tanks were common, most radiator caps had a metal seal between the outer ring on the radiator neck, and the cap. This would not hold vacuum, so there was also a different type of cap with a rubber gasket that sealed the outer part of the radiator neck, for use with recovery tanks. Over the years as all new cars got recovery tanks, they quit making them without the rubber seal...all the new caps have this feature.
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