[camrycurt]

Originally Posted by 68gmsee

Upper seal keeps coolant in the system from escaping out of the top and basically keeps it an enclosed system.

Bottom seal opens up when radiator pressure exceeds cap pressure rating. Heat expanded coolant is then vented into reservoir.

When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled. That would be the brass "check valve" in the bottom center of the cap.

I see. So it's the addition of the brass check valve (and not the extra rubber seal) that differentiates the caps. That makes sense.

Thanks for clearing that up for me!