Don't celebrate too soon grasshopper. There should not be a vacuum in the system, because that is one of the jobs of the coolant cap. Can't call it a radiator cap anymore, because these days it is seldom found on the radiator. A coolant cap has many functions. First it keeps twigs and whatever out of the system. Secondly it keeps the proper pressure on the system to prevent boiling. Thirdly it keeps coolant from escaping from the recovery tank during cool down like Brian said. Lastly it keeps the system from going into a vacuum during the cool down process. How coolant escapes during the cool down process is another lesson. Hopefully you solved the problem. Keep us posted.

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