Radiator cap Question

I just replaced my rad. cap about a week or two ago. I did this because I noticed that it was damp with coolant around the filler neck on the radiator. The cap that came off was a 10# as is the new one. This new cap didn't solve the dampness around the filler neck of the radiator though. This evening after I shut the truck off I open the hood and I noticed that I have the dampness around the filler neck again, but I could also hear air coming through the overflow line (just air no coolant coming out) Keep in mind this is with the ENGINE OFF. I don't think this is a head gasket cause the radiator doesn't have any bubbles or steam in it w/cap off and engine running. I have no problem taking the cap off the rad when the engine is hot and not have any more psi than normal. I did notice that the new cap didn't have as wide of sealing ring on it (rubber ring on the bottom of cap). Do u suppose that might be the reason the dampness is showing up on the filler neck? When the coolant gets hot and expands the cap doesn't seal so it leaks a little coolant out when the engine is being ran and when u shut the engine down at the end of the day the system being expanded and the cap not sealing to the rad. neck is letting the air out the overflow. I hope this makes some kind of sense.

 

Just thought I chime in a let people know that this issue ended up being a blowed out head gasket and a dropped liner. All is good now did inframe and cut the counterbores.