I was looking at some radiator caps, I have laying around, and noticed I had some older ones with no seal directly under the cap. Now from what I remember there are two types of caps: Vented & NON-Vented. First off, NO, a pressure relief lever on the top of the cap, does not make it vented and NO, I am not confusing gas caps with radiator caps. As I understand a Vented cap or Closed System cap means, as the system cools down, it will pull fluid from the ZERO PRESSURE recovery tank. Closed meaning fluid is not released to the ground via an overflow tube. This cap will have 2 rubber seals.<O></O> A Non-Vented cap or Open System cap- means it will not pull fluid back into the cooling system. Open means when the pressure gets too high it releases onto the ground. This type of cap will only have ONE seal on the brass pressure valve disc. Does everyone confirm this? I have seen guys add recovery tanks to their rides without changing to the proper cap. I'm sure running a vented cap on a system without a recovery tank is okay but it would pull air into the system upon cooling. <O></O>
Here is what Wiki Answers has to say about it: Look at the bottom of your radiator cap. There are two seals. One that seals at the top neck and one that seals at the bottom of the filler neck. As the fluid heats up it expands. It eventually over powers the primary spring in the radiator cap. The bottom seal is broken as the pressure exceeds the spring force of the cap and the cap rises. Fluid then enters the overflow tank until enough pressure is relieved to allow the spring to force the bottom seal back into place. You'll also find on the bottom center of the cap that there's a brass or stainless circle plate. That circle plate actually creates a one way check valve. When the engine cools back down the coolant shrinks in volume. As that happens the pressure in the coolant system becomes a vacuum pulling down on the bottom circle plate and thus opens the check valve. Once the valve opens the fluid is drawn in through the tube that leads to the bottom of the reservoir and refills the system. The system is actually self burping in that it evacuates air on each temperature cycle. Air is pumped out when the system gets hot and fluid is then drawn in when it cools. Don
So the purpose of the top seal is to prevent fluid escaping out the top of the cap as it is pulled back into the radiator. Thanks Guys