Engine overheating

Hi its my first post I have a 1972 mercedes 230/6
the radiator pipe bust yesterday and it over heated right up to the red now I repaired the pipe an filled with water I left the cap of and started it the water runs out of the cap.I know thats not good news as it might need a new gasket an head pulled off. But a friend of mine said that I must bleed the water system?I noticed on the thermostat housing ther is a nut now how do I bleed this system i'v never heard of this before please can you give me some info as I dont want to pull the head off. Thanks in advance