Possible cause...
 

...of a collapsed upper radiator hose? Hose was replaced in 2012 when they got my 1973 Firebird Formula running again. Original radiator still, original cap. Was in normal shape after coming back from the show.

Cap may not be letting air back into the system when it cools down. Pulls a vacuum slightly on the system. I see it allot in the trucking industry

I had the same issue when I purchased a repro cap from a well known Vendor.

Went to the chain autostore and bought a cap for a quarter of the price from the above vendor. No more collaped hose. It seems the repro cap isn't letting in air as stated by Chiefbigb.

Be sure to wipe out any crap from the sealing area that the rubber of the cap mates to otherwise even a good new cap might produce the same issue.

I was already thinking cap and checked it out. There was no build up. Rag turned black where wiped from cap though. So, that will probably be replaced.

Interesting. I had this happen the other day and as it happens recently replaced the radiator cap with a new repro.

Verdero,

That is very interesting. I purchased mine a few years back.

When I compared the repro one to the chain store one I notice the little vent/check valve built into it seemed very tight compared to the other one. Maybe a flaw in design?

This one is super tight and very hard to twist closed. I thought that was a good thing, but maybe not.

The hose hasn’t collapsed again since that one time, but I've been checking every time I drive it.

We've had several 'new' caps that didn't work correctly.

When you took the cap off, did the hose immediately return to its' normal shape?

I didn't take it off at the show, left in a collapsed shape. And was back in shape when I got home, even after the cool down.

I don't think there is a negative effect to the condition other than the visual side once the system has been run and air purged from the system

You figured good on the cap. Add more coolant. Run a catch can. Be happy.

It seems the rubber products coming from China are inferior and this has caused a multitude of problems in recent years, and may be related to radiator cap issues.

I would also check the hose going from the radiator to the overflow tank. Make sure it’s not collapsed or full of junk. Same goes for the tank. It all needs to be free flowing.