Start by putting a piece of tape on the coolant over flow tank to mark the coolant level when the engine is cold. Then start the engine up and bring it up to operating temperature. If the level doesn't rise, it means the pressure cap isn't venting excess pressure (coolant). If the system is full of coolant, the excess has to go somewhere when the engine warms up. If the level doesn't rise when the engine warms up, and the coolant level is full inside the engine when it cools back down, the heat exchanger is where I would start. When was the last time the zincs were changed on the heat exchangers, and what was left of the old ones?
Ted