Rad Top Hose Collapsed

[AntStark]

Just opened the bonnet to check everything was ok after a weekends driving in the sun. I noticed my top radiator hose was as flat as a pancake. I undid the radiator cap and there an obvious vacuum as air rushed in and the hose expanded again. I assume these are all the symptoms of a duff radiator cap and I just need a new one ? On the upside I guess it proves everything is air and therefore water tight. Coolant levels are also spot on.

 

[Kiwipom]

My gut feeling is everything is fine, its a sealed system. Only if it gets over pressurised will the cap release gas/fluid to atmosphere. Possibly topping the reservoir to max will reduce the vacuum when the engine cools.