water comming out thru top!

my question is that if it is regular for water to get filled up all the way to the top in the reservoir and i mean to the top not where it says max in a 91 sentra se-r? (the reservoir that is connected to the radiator and next to the windshield wiper water reservoir), it fills up but the temperature gauge never pases the middle so i think it is not hot and some water comes out thru the top. do you know if this is a problem or is this regular?