Don't know what year your E38 is, but if it has the quick connect ends then this is a common problem of the hose coming off the plastic ends. It was in my case. I just solved the problem by changing the hoses. If they haven't ever been changed, change them. And be ready for other cooling system components to fail shortly after if they are old.
color: anthrecite metallic
wheels: style 89 E66, 19X9F 19X10R
ext. lighting: 8000k D2S & LED plate light
*want to get sport rear diff*