What would cause a drip at the spirovent? Just noticed a drip. Thanks

I think I know the issue, but wanted to get some insight. I have a single zone, hot water baseboard heating system. In the basement, it originally had 1" copper that reduced to 3/4" then finally to 1/2" once it finally branched off to the area of the house it was heating. I replaced all the basement copper with pex and used all 1" to 3/4" in the basement and installed new 3/4 baseboard on the first floor. So everything from the basement through the first floor loop is 3/4". The copper going from the basement to first floor is 1/2" and throughout the 2nd floor baseboard it's 1/2" back down the the return. I am not changing the copper running up to the second floor because I cannot access it without major renovation. Here is the problem. After I changed it all out.. I no longer have heat on the 2nd floor loop. Copper barely gets hot.. But I can hear water flowing when I turn the heat on. Is the change to all 3/4" on the first floor causing a massive reduction in flow to the 2nd floor because most of the water and flow is diverted to the 3/4" first floor loop? If so, could I just splice in a small section of 1/2" at the end of the first floor loop to restrict the flow to the entire floor and hopefully balance out the flow between the floors? I know that seems ridiculous to restrict flow but not sure what else to do if this is my issue and replacing 2nd floor run piping is not an option.