Did you ever do any canning? Canning produces a vacuum all by itself. Your radiator uses the same technique; first coolant is heated but when it cools down, the air inside contracts into a vacuum (provided your radiator cap has a good seal).
Cars with radiators LOWER than the engine (like Corvette and Escort) have used this technique for decades. It's important for those cars to have coolant in the aluminum heads at all times or they crack.
Over a few heat/cool cycles, your heater core should displace the air bubble with coolant. This is evidenced by finding a lower coolant level when the system is cool enough to check. Modern cars use the overflow to pull liquid back into the system. That's why their systems are always 'to the top'. - Dave
Cars with radiators LOWER than the engine (like Corvette and Escort) have used this technique for decades. It's important for those cars to have coolant in the aluminum heads at all times or they crack.
Over a few heat/cool cycles, your heater core should displace the air bubble with coolant. This is evidenced by finding a lower coolant level when the system is cool enough to check. Modern cars use the overflow to pull liquid back into the system. That's why their systems are always 'to the top'. - Dave
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