Overflow bottle

The old radiators without a puke tank had a one way cap on them. They puked out coolant and it was over. Sealed until next time.
The later systems with puke tank have a two way cap so that as the system cools down, it creates a vacuum and draws the coolant in the tank back into the radiator. It actually has two seals in it.
I had some pics of the different caps that I have posted before. I'll see if I can locate them or you can Google two way radiator cap and it can show you more. Dennis