Cadilac Catera Problem NEED HELP

I want to know what someone technical can help me and tell me what they think would happen If you run this car with the radiator cap, (it is
located on the overflow tank) with the pressure valve missing out of the cap and only a gasket inside of the cap and put on the tank air tight. What is the purpose of this pressure valve and what damage could it do to the car running it without it? Thanks Can email me at
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