When I bought my Chevelle 8+ years ago, the original AC RC-15 radiator cap still functioned perfectly but it had a small rust pinhole on the top. For purely aesthetic reasons, I replaced it with a reasonably correct appearing reproduction, but kept the original. Yesterday, I was checking various fluid levels on the car and removed the radiator cap to check coolant level. When it came off, I noticed all this rusty scaly crud in the neck of the radiator. Looked at the cap and was unpleasantly surprised, to say the least....
Both the spring and the piece between the spring and the brass plate that seals againt the opening in the radiator neck were very heavily corroded, as you can see in the first picture. Many of these bits of corroded metal that broke loose can be seen in the second picture. You can also see the rubber ring that had been attached to the brass disc (to help in sealing) had completely detached from the brass disc.
Luckily, I still had the original cap (stored in the box that the repro came in, which you can also see in the pictures) so I dug it out and pressed it back into service. It still works fine, and has not one trace of corrosion in the 'working areas'.
If you have one of these caps, you might want to check it periodically to make sure this doesn't happen to you!