1977 240 overheating
 

i put a new radiator cap on, and drove it for 30 miles card did great. then shut it off, let it cool down. and drove it again but it overheated again all the way to the red in less than 2 miles. there is a small leak at the bottom of the radiator from a plastic screw looking plug. is that normal...please help i need to get to school and back.