The CAC is a radiator just like on the front of the truck except the heat is travelling the opposite way. At the radiator, heat from the coolant is transferred to the flowing air via the fins. In the CAC, the hot air from the turbo passes through the fins and transfers heat to the IC's coolant. The air should be contacting the metal, which contacts the coolant. If it's leaking you definitely have a problem, though I'm not sure what would cause it or if it's even possible to fix it...