I own a 2002 Land Rover Discovery Series II. In the morning before starting vehicle, I noticed my upper radiator hose is collapsed/ a vacuum has occurred. I opened the coolant reservoir cap and the vacuum was released. What causes this?

 

I would start with a new rad cap and go from there. Are you losing coolant?

 

Purchased the cap. Not losing any coolant

 

Same problem -> bought a new radiator cap -> problem continued -> bought another radiator cap -> problem solved.