1998 Bmw M3

I have a question, I just replaced the water pump, fan clutch, all the belts, all the hoses, thermostat, and housing (water outlet). After a week or so, driving with the air conditioning on, when car turned off, there has been a hissing sound at the top of the water outlet and some coolant is coming out of what looks like the upper bolt on the outlet. The bolt is super tight. Also on the resevoir tank, the sensor is broken. Would this cause fluid to leak out, because I have to check it everyday day and fill resevoir. I put the chrome water outlet on, over the plastic one from factory. Any ideas. Also looking with a mirror yesterday, looking from bottom looking up on the water outlet, in two areas you can see a little part of the gasket. Would that cause the leaking, it looks flush and tight? Thanks for help. Is there a diagram of the cooling system, water outlet etc