2. the purpose of the radiator cap is to actually hold pressure in your cooling system. However, if that pressure gets to high your radiator cap has a spring between the housing of the cap and a rubber seal. When the pressure exceeds the force of th spring, the spring compresses and the rubber seal lifts off the top of the radiator spout. after this seal lifts there is a small hole in the radiator spout just above where the rubber would make the seal. that hole has a hose connected to it from the outside and goes to an over flow tank, this way extra fluid can escape thus reducinc volume in the system and consequently pressure as well. hope this makes sense, if you can't visualise it pop your hood and go take a look at ther parts.
I would recommend replacing the cap if it looks worn, the rubber seal is dilapidated or if its really old. make sure you get a new cap that has the same spring force. it usually says on the cap for example: 13lb or something like that
1. i never have and i havent had any sealing problems. just tighted it down decently but be carefull not to over tighten
