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Hello, my name is***** on description it sounds like head gasket is leaking. The head gasket will pressurize cooling system if head gasket leak which is why system pressurizes right away. To verify this is the problem you will want to do a cooling system pressure test, most parts stores will barrow out pressure tester for free. If cooling system does not hold pressure but no external leaks you know the head gasket is leaking. Let me know if there is anything else I can do to help or if you would like replacement instructions. Thanks.