C15 6nz Coolant in fuel separator

[flonly]

I had a 3406 e in a 98 t600 that had injector cups leak 2 different times and both times it was pushing diesel out of raditor overflow tube onto ground.I was told the first time that when the motor is running there is more pressure in i guess the fuel system so fuel is pushed into raditor and when you shut truck off the raditor still has pressure so you then can get coolant into your fuel.