Sorry for being unclear. Here's what I mean: in the pic I attached, you're looking at the underside of a radiator cap and a filler neck. The radiator cap has two seals, one outer, which seals against the top of the filler neck, and one inner, which seals against the lip on the inside of the filler neck. It was mentioned earlier in the thread that older radiators often had taller necks, which meant that using a modern cap on them will allow for a seal against the top of the neck, preventing leaks, but won't seal against the lip (because it won't reach down far enough to touch it), which means that the radiator won't be pressurized. The inner seal is pushed against the lip by a spring (seen between the inner seal and the top of the cap), and the spring strength determines the pressure rating of the cap -- the stronger the spring, the more pressure it takes inside the radiator to force the seal up and off the lip and allow coolant to escape and travel to the reservoir.
As for returning from the reservoir, see that metal button-looking guy right in the middle of the cap, centered in the inner seal? That looks like it holds the whole thing together, but is actually a return valve. If you pull gently (gently!) against that button, pulling it away from the seal, it will pull out against a much weaker spring and open a passage so coolant can bypass the inner seal and get pulled into the radiator from the neck (fed by the hose from the reservoir). However, I believe that some of the older radiator caps did not have that ability -- they did not have a built-in return valve. So I was recommending that A) you verify that your cap reaches the lip and is long enough to pressurize your radiator, and that B) that you use a modern 7lb cap that allows coolant to be pulled back into the radiator through the valve that's centered in the inner seal.
Hope that helps!
