Question About The Overflow Bottle For The Radiator.

Hello,
Short story about my 5.0 overflow bottle. It has never really latched tight even when I bought the car a few years ago. I have to end up adding some water through the summer. This is my boat, the convertible only comes out sunny days with no rain in forecast so not a huge problem. I had a boil over when I drove it a far distance for the first time this year. I finally decided to replace the overflow bottle so the cap would fit tight and I thought keep from having to add to the system. And flush the system because I think I'm real low on antifreeze at his point. The new bottle arrived and I tried it with the original cap and sensor and the fit is no tighter than before. I also noticed that the bottle has two outlet ports, the bottom one is easy to see that it connects to the radiator but the top one turn back down and doesn't seem like it connects to anything. The current one in the car is the same way. Is this a blow off port or should it be connected to something? If it is a blow off port I guess the cap tightness is not as important as I thought. My VW is a closed system so that's what I thought this would be.

Thank of reading and I hope giving me some info. On this in advance.

Thanks for the replies. That makes since now. I guess the tightness of the as important as i originally thought. I'm going to flush the system and restore the normal balance of coolant and i guess i should be good. Other than this overheat I have had no problems for the 3-4k i put on it in a year. thanks for saving me the cost of a new sensor cap to see if that made for a better fit.