When I turn my engine off & let it sit for a while the upper radiator hose appears to collapse
from an internal suction. Is this normal, or the manifestation of an internal vacuum leak? :confused: Thanks

Sounds like not enough water in the system and a week hose.

Are you using a recovery/overflow system?

It will prevent that from happening, when using the correct radiator cap, and a tank that's vented to atmosphere.