2002 9400i C15 6NZ cooling system capacity

I asked this in another garage forum and now asking here wjth hopes of some others reading it. 2002 Int 9400i w/c15. What is the cooling system capacity for this truck? When I drained it, all I got out was 7 gal. It would only hold 7 gal at refill. It sure seems like it would be more. Am I missing something here? Blocked radiator? Air block? Truck does not overheat but does push coolant out of overflow tube. Ideas welcomed​