I just don't get it..

You just MIGHT have a wrong cap on there---there are dfferent neck lengths. I don't know how many, I think there's at least three

I think I'd be swingin around a radiator shop and see what they think.

If the engine is cool, you might be able to squeeze / release the top hose and feel it "breathe" because some of the check valves are not really spring loaded.